As George points out, using material with the burn certification is the only item that needs to be understood by whomever you choose. Using people like Shelby at TN Wings who understand the regulations regarding burn certs and log book entries is a safe way to go. Shelby also knows where to look for problems (cracks) in the seat frames when he has them stripped down so that's another plus.
I used a local shop that specializes in custom hot rods and show cars to do my upholstery and they sourced the aircraft grade material (Ultra Leather and wool carpet) from their vendor so I have the necessary documentation which the vendor supplied.
Custom interiors are quite special especially when we consider how much time we spend inside the cockpit.
