Hmmm. That
is tough.
About the only other option I can think of right now is that you try to get hold of a professional restorer - someone like John Wyatt (Rising Sun Restorations). Doesn't have a website, but his contact details are
here. If anyone knows where to get one, he will, although I've no idea how/if he responds to requests for assistance.
Alternatively, I s'pose you might be able to track down one of the replacement seat manufacturers and ask them if they can get you a cover . . .