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The Erenow site has an image of the seal on it's page titled The Fears of Henry IV, although it isn't particularly high resolution, and doesn't provide further details for the source of the image:
This seems to match the description that you mentioned on p 242 of Charles Boutell's English Heraldry
"... a very remarkable Seal, used by HENRY IV. a short time before his accession, the shield with helm and crest are placed between two tall Feathers, about each of which is entwined a Garter charged with his favourite and significant Motto the word SOVEREYGNE, ..."
I also found the records of two seals in the card index to armorial seals & seal impressions held by the UK National Archives (QFA 1-25). These describe the ostrich feather
"... encircled by a scroll lettered ma/so/ve/rey/ne"
and the cards date the seals to 1394 and 1395.
You would probably have to visit in person to view the actual seal impressions.