A lot of your racecar manufacturers offer seats. But if it's like mine you may have to find an aluminum fabricator or dragster builder to get one to fit right.
If it's like mine, it's a hand made aluminum dish more or less. It's like what's in a dragster. But It would seem to me to be a lot of variables as far as buying a pre-made one. Chassis width and height to shoulder bar being 2 of them. There's also the angle or how much the seat lays back. I've seen them sit up straight and ones that you almost lay on your back to drive.
Yes, that is the problem, I am sure it is going to have to be custom made. The height and width will not be standard. Just thought I would ask and maybe get some ideas. THanks