I agree with Vic, I found a set for my 39 ford on eBay. The box said western auto I think. They were nos. didn't fit perfect but really well for being a supplier brand. I would check on there often as they do seem to pop up from time to time. I paid 125.00 for mine.
That has been my experience with both NOS and NORS covers. Not 100% certain what they made them out of, but man, they sure do hold up!My seat covers were red with a plaid insert. They were NORS as stated above. There was no rot, and in the sun the shape and feel came back. I drove it for over 4 years with them and they still looked and felt new.