Couple tidbits to pass along...
-- Try using the term cockpit cover instead of transom cover if you're searching. I don't think I've ever heard of the term transom cover for this since it's not really accurate.
-- Keep in mind that if you buy a used cover you will need to move the snaps on the cover to align with the snaps on the boat. Unless you don't have the snaps on the boat, already, of course.
-- Try RNR Marine, Great Lakes Boat Top or AJ Canvas
-- A local shop can make it for you, including a full cover
Good luck!