There are people who still make them. A local artist who does ambrotypes had one made to fit his 10x8. It holds various size plates form 10x8 down to 1/4 plate and smaller.

The place to check would be those civil war reinactor groups as they have people who do this sort of stuff (why call any war civil, sounds like an oxymoron to me...any way back to the subject).

If you have no luck, let me know and I can track the person down and find out who made it for him.