Is it OK to purchase cat food?

May be you could feed your cat egg products ? It minimizes the negative karma yet may also be a nutritionally complete meal. I don't have any cats and not sure they would consume eggs, but you can try feeding the cat part of your omelot. You can also try try tuna which may have less of a karmic repurcussion than something higher up on the food chain. It's been said the difference between killing a plant and killing an animal is that you can't hear the plant scream. Keeping this in mind you may want to choose a diet that minimizes the withdrawl from you and your cats karmic accounts.

Vajra Yana Buddhism ( Ref-Wikipedia) allows the followers to take meat and the Preachers ( Acharyas) to get married .
It came as a variant of Buddhism in India ,firstly as an attempt to combine Tantra System of Hinduism with Buddhism and later around 7th-8th Century AD to justify the use of Violence for Self-Defence and for protection of Buddhists from opponents of Buddhism --mainly Islam which justified the use of Violence for spread of Religion.
This form of Buddhism spread to other parts of the World and is known by different names in different regions.

I also understand that eating meat (something I admittedly still do myself) is frowned upon

It depends on the tradition and personal beliefs. If you follow Theravada Buddhism, you wouldn't have any issue with feeding yourself or your cat with meat products. In Theravada Buddhism, the word of the Buddha is valued higher than personal opinions. So you or your cat is less likely to get frowned upon, just for eating meat. :) Vegetarianism is usually treated as a personal food preference.

As noted elsewhere, it's pretty much only Mahayana Buddhism (especially Chinese, Korean, Vietnamese Buddhism) where vegetarianism as an ideal is the norm. So I'm answering from that standpoint.

The Upaseka Precepts Sutra* does have something to say about pets. They essentially promote divesting of your animals, specifically singles out carnivores as unallowable animals to own, and suggests releasing them back to the wild. In practice, some lay followers donate the animals to temples, which let the animals live out their life on the monestary.

I'm a vegan, but out of accident of family history I own a cat, who kills mice in my apartment and eats cat food from a bag, the label says it is 20% plant matter, so which is 20% more than a cat would eat in the wild. Obviously I'm a pragmatist and prefer harm reduction over harm rationalization.

A random observation is that my cat loves corn. Some of the cat food brands he has eaten use corn gluten as the source of protein and he really likes the flavor of corn.

• An Upsaksa is a lay person, this sutra is for lay people who takes on more than the 5 precepts, but fewer than the Bodhisattva Precepts or the vinaya precepts, which are for monastics.

ref: http://www.sutrasmantras.info/sutra33c.html

(16) If an upāsaka who has accepted this precept raises animals, such as elephants, horses, cows, goats, camels, or donkeys, and refuses to give them away to someone who has not received the [upāsaka] precepts, he has committed the sin of negligence.

ref: http://www.ic.sunysb.edu/Clubs/buddhism/bns/bnstext.htm

32. On Harming Sentient Beings A disciple of the Buddha must not sell knives, clubs, bows, arrows, other life-taking devices, nor keep altered scales or measuring devices. He should not abuse his governmental position to confiscate people's possessions, nor should he, with malice at heart, restrain or imprison others or sabotage their success. (82) In addition, he should not raise cats, dogs, foxes, pigs and other such animals. (83) If he intentionally does such things, he commits a secondary offense.