I would suggest talking to an upholstery shop or to either MII Specialties or Second Gen. This is an item that will require some expertise to locate.
TMI (
http://www.tmiproducts.com/) makes seat foam and upholstery for the early Stangs, but the @#$!'s don't make anything for our cars. You should register a complaint with them.
I'm curious how close a '65-'66 seat is to a MII seat...its been years since I sat in one of the early seats, but they certainly "look" a bit like ours...other than that chrome side trim anyway.
The reason that I bring it up is that TMI has a new seat foam and upholstery set for the early Mustangs that has pronounced bolstering to hold you in the seat better. I could sure use that in my MII!
