Several usual suspects sell seat covers for GT6 highbacks. Try SpitBits first, they're specialists and carry lots of Spit/GT6 parts.
I've never seen reproduction/new seat rails but I'll bet you can find some used. First, I'd try the NASS Yahoo groups board, those guys will help. Also, I suggest that you join NASS, a really knowledgeable and helpful bunch of gearheads. They also have a nationwide network of folks who can help. And, always check with Ted Schumacher at TSI Imported Auto. Top drawer guy, sometimes even if he can't get the parts for you he can point you in the right direction to get them.
Here's TSI:
https://www.tsimportedautomotive.com/
Seat covers are expensive, you won't find any cheap ones anywhere that I've found. In fact, it may be more cost-effective to have your seats recovered at a local upholstery shop. Good upholsterers can mimic the stock covers, maybe at a better price. It's worth getting an estimate.
Sourcing parts for moribund antique British cars is just part of the joy, isn't it? With a bit of research you can find practically every bit for a GT6.
Good luck, let us know. :thumbsup: