I just use extra scraps of mats (leftover from framing projects). It should be non-acidic, but others on the board here can probably recommend what specific type of backing is best. I'm not nearly as anal/obsessive about it as some seem to be.
When I receive photos from Henry Yee and other sellers, it just seems to be that smooth white type of cardboard (placed inside of a plastic sleve). Sorry, that's all I've got
