In North America you can get 48x96 inch mat boards but only in white and offwhite from Bainbridge through your wholesale frameshop. The rest of the world you can get 48x96 black also. If you want a black mat, you have to use a velvet mat and cut it at a diagonal but the ends together and rough over the velvet so the line disappears. Velvet mats are quite a bit more expensive but a 48x96 mat will be expensive also. I have recently looked into this as my prints are up to 30x90 inches.

Darcy