I tried to get some for my aging Momo Acropolis seats and was met with unanimous
"we'll call you back with that..." No vendor ever did, including Momo in Italy (I finally called them as well.
I expected to be able to find replacement covers when I first started to look because those seats were $750 each, which was a lot in 1994. Come to think of it, it still is.
It makes sense from a safety standpoint. The flame retardant fabric loses it's ability to resist burning and the shells get brittle. That's why the FIA ratings expire.
You can buy Kirkey and always get new covers for their seats. I may go with them next time.
http://www.kirkeyracing.com/Good luck