Eric,
The seats in my SPF GT40 #2182 are pieces of upholstered dense foam that sit in the monocoque chassis "seat trays".
The upholstery is a dark grey "Alcantara" fabric. You can purchase this exact fabric from Gulffabrics.com. (I know, because I'm making some matching interior sill covers for my car).
I wouldn't think it would be too hard to craft your own seats that would match those found in the SPF. (And much cheaper, I suspect).
Jack