What is the source for this quote from the French Revolution?

I'll contest that this is really a quote about Marat.

Just answering the question title: The source of this quote is a monsieur Vergniaud.

Marat acquired a dark legend of being blood thirsty. But it was probably Georges Auguste Couthon at whom this accusational quip was originally directed as well:

The Commission of Twelve became the pretext for the coup d’etat that took place between May 31 and June 2 of 1793. The mystery is that the Girondins had lasted as long as they did. The scene which took place that June day in the garden of the Tuileries and the subsequent scene within the palace, when Marat contemptuously read off the names of the men who were to be expelled from the Convention, brought down the curtain on one act of the Revolution. […]

Broken and humiliated, the Convention on that day ceased to exist as a representative body. The expelled deputies were put under house arrest, a liberality that poorly disguised the Commune’s ultimate intentions and that provoked one of the victims to cry out.
"Give Couthon his glass of blood; he is thirsty.” They were not under any illusions, and during the night of June 2-3 certain of them, in disguise, made their way out of Paris.

— Stanley Loomis: "Paris in Terror. JUNE 1793-JULY 1794", Jonathan Cape: London, 1964. (archive.org, p 105)

This was indeed spoken by Vergniaud. The Law of 22 Prairial was drafted by Robespierre and Couhton, and when they presented it in the convent Couthon justified the law with:

Couthon proposed the law without consulting the rest of the Committee of Public Safety, as both Couthon and Robespierre expected that the Committee would not be receptive to it. The Convention raised objections to the measure, but Couthon justified the measure by arguing that the political crimes oversaw by the Revolutionary Tribunals were considerably worse than common crimes because "the existence of free society is threatened." Couthon also famously justified the deprivation of the right to a counsel by declaring that the guilty have no right for a counsel and the innocents do not need any.

During the convent Couthon was a speaker, Marat only sitting on the side on an emporium. It appears that while Couthon was speaking, Marat drafted a list of whom to accuse next, including Vergniaud.

In Paris itself, however, the maximum number of executions took place in the spring and early summer of 1794—just as they were declining in the provinces. Of critical importance in this turn of events was the so-called Prairial Law (June 10, 1794), streamlining trial procedures in the Revolutionary Tribunal of the capital. Robespierre closely collaborated on the law, though it was ultimately written by Couthon. Couthon made his goal quite clear: “It is not intended to make a few examples, but to exterminate the implacable satellites of tyranny.” The most novel portion of the decree was the set of remarkably elastic clauses de ning those to be considered as the “satellites of tyranny”: anyone who attacked the Convention, betrayed the Republic, interfered with provisioning, sheltered conspirators, spoke ill of patriotism, misled the people, spread false news, outraged morality, abused public o ce, or worked against the liberty, unity, and security of the state. Henceforth suspects would be allowed no defense attorney, there would be no preliminary hearings, and the only possible verdict would be acquittal or death.

— Timothy Tackett: "The Coming of the Terror in the French Revolution", Harvard University Press, Cambridge, London, 2015, p322.

In fact, many people were accused of being blood thirsty, and many speeches mentioned blood that was shedded or had to be shed… I personally doubt that any book about the Grand Terreur can manage to print the word "blood" under a hundred times.

Vergniaud seems even to have made this a symbolic trademark of his:

Still with unfaltering courage they continued their resistance to the dominant faction, till things came to a head on 2 June 1793. The Convention was surrounded with an armed mob, who clamoured for the "twenty-two." In the midst of this it was forced to continue its deliberations. The decree of accusation was voted, and the Girondists were proscribed. Vergniaud is noted for his last gesture of defiance in standing up among the subdued deputies and offering them a glass of blood to slake their thirst, a metaphor for their betrayal of the Girondins.

In many books many variations are found.

The French Wikipedia gives yet another variation:

À Marat, devant l'assemblée législative: "Donnez lui un bon verre de sang pour le rafraichir!"
(Giving as source: Philippe-Joseph-Benjamin Buchez, Prosper-Charles Roux, Histoire parlementaire de la Révolution française ou Journal des assemblées nationales, History, 1837, pp. 457)

As this is an official parliamentary archive record. That seems to indicate Marat as target. Unless Vergniaud repeats himself, as on June 2 he said:

A ce moment l'orateur demanda qu'on lui portât à boire. –– "Couthon a soif, s'écria Vergniaud, donnez-lui un verre de sang." Maintenant que vous reconnaissez, conitnua Couthon, combien vos dèlibérations sont libres, je demande, non pas quant à présent, un décret d'accusation, mais arrestation contre les vingt-deux membres dénoncés, contre les douze de la commission et contre les minstres Clavière et Lebrun; ils seront, non pas détenus, mais en arrestation chez eux."
__At this moment the orator asked to be brought to drink. "Couthon is thirsty," cried Vergniaud, "give him a glass of blood." "Now that you recognize," cried Couthon, "how much your liberations are free, I ask, not for the present, an indictment, but an arrest against the twenty-two members denounced, against the twelve of the commission and against the minstras Claviere and Lebrun; they will not be detained, but will be arrested at home."

— Amable-Guillaume-Prosper Brugière Barante (baron de): "Histoire de la Convention nationale, Vol 3" 1851, p160.

I conclude from the variety of attested quotations – including those found in @Evargalo's answer – that this was not a single incidence. As Evargalo commented: "If Vergniaud was happy with the effect he made with that sentence in the Convention, there is a chance he re-used it 50 days later during the riot. In that case, both Marat and Couthon would have been targeted."

It looks like this was Vergniaud's method of protesting and polemicising against the violence and those who demanded it. Like a truly modern politician, he likely repeated that phrase. In that sense his Make the Revolution Great Again is a slogan directed at quite a few people at different times.

This view might be reinforced by the legend at the time of l'héroïne au verre de sang, Marie-Maurille de Sombreuil connected to a man of very vile reputation for the massacres he oversaw, and for what he is said to have done to a young woman:

WP: Stanislas-Marie Maillard:
While serving as president of the improvised tribunal at the Prison de l’Abbaye, he released the marquis Charles François de Virot de Sombreuil, who had been saved by his daughter Marie-Maurille, to whom legend confers the status of l'héroïne au verre de sang. This name refers to the legend that, in order to spare her father’s life, she was compelled to drink a glass of blood. Jules Claretie, in the role of second-in-command, gave an eyewitness account of Maillard in the role of judge: “Maillard was a young man of thirty, large, dark, with matted hair. He wears stockings, and a grey habit with large pockets.

An incident to which Vergniaud apparently alluded to in public.

So the sources seem to indicate that he used this picture speaking about Marat and about Couthon:

Elected deputy, sitting at the top of the Mountain, president of the club of the Jacobins since April 5, 1793, Marat becomes every day more formidable, accusing, slandering, insulting, belching.

"Give a glass of blood to this cannibal: he is thirsty! "1496 Pierre Victurnien VERGNIAUD (1753-1793), to Marat vituperant at the tribune of the Convention, April 13, 1793
Famous proceedings taken from the Essay on the General History of the Courts of the Peoples of Old and New Nations (1796), Nicolas Toussaint Le Moyne Des Essarts.
Elected deputy, sitting at the top of the Mountain, president of the club of the Jacobins since April 5, 1793, Marat becomes every day more formidable, accusing, slandering, insulting, belching. No one seems to be able to interrupt it - let us note how much blood, word and symbol, is present in this story.

Marat preaches the spiral of terror and the logic of hatred, for almost three years now.

And Couthon, as shown, June 2 1793:

No, it was not the lives of the Girondins that the revolutionaries wanted. Vergniaud, in one of those tragic days, exclaimed: "Give Couthon a drink of blood; he is thirsty". Vergniaud was mistaken, Couthon was not thirsty for blood. But the Gironde had become a mortal danger for revolutionary France. She must disappear. On June 2, its political power collapses.

— (Variation in Histoire des Girondins, Vol 6)

L’Histoire en citation mentions this quote:

Donnez un verre de sang à ce cannibale : il a soif !

which I would translate as:

Give a glass of blood to this cannibal: he is thirsty!

It is attributed to Pierre Victurnien Vergniaud addressing Marat at the Convention’s tribune on the 13 April 1793.

Their own source is:

Procès fameux extraits de l’Essai sur l’histoire générale des tribunaux des peuples tant anciens que modernes (1796), Nicolas Toussaint Le Moyne Des Essarts.

I found the original text here. Since this source in from 1796, I think it may be the oldest reference of the quote we can find.

The exact quote would actually be:

Donnez un verre de sang à ce cannibale : il en a soif !
(Give a glass of blood to this cannibal: he has thirst for it!)

The exact date is not made more precise, but it is described as an interruption by Vergniaud during a speech of Marat at the tribune.

The 13 April 1793 is credible, since Vergniaud delivered a noted speech on that day to expose the conspiracy of Montagnards and denounce the revolutionary tribunal. At that time, the Girondins and their leader Vergniaud were already targeted by Marat, Hébert, Robespierre and the revolutionary tribunal. They would be guillotined the 31 October 1793 – providing more than a glass of blood, yet not enough to tame the thirst of the tribunal.

@LangLang's answer provides an occurence of Vergniaud using the quote a few months later. My best guess is that Vergniaud, like most politicians when they are happy with a tirade, used it several times.