I don't believe the cover alone can be purchased, at least not from New Holland. My 1920 had a rip in the seat when I purchased it, so I checked with NH parts about getting it replaced. Turned out I was looking at replacing the entire seat pan for $200. I decided to live with the rip.
It it's past the live with it stage, check into generic aftermaket covers, or, as suggested above, an upholstry shop.