Agri supply has a lot of them as mentioned.
However, I'm in about the most remote corner of buttcrack Mississippi that you can imagine and I can think of about three upholsterers within a 30 mile radius. I'll bet you've got more than you know near you if you ask around a little.
I'm friends with one of the two local body shop folks here in our neighborhood. He sends his stuff to the "big town" 100 miles away. I do know where a saddle shop is, maybe he could help. Leather seat wouldn't bother me a bit. Thanks for joggin' my memory,