Any serious art store should have diffusion sheet. I prefer 3-mil and 5-mil mylar frosted on both sides. The sheets are about two by three feet or so. After I get them to the lab I inspect them over
a big light box, then cut things down into slightly oversize 4x5 or 8x10 film sizes. Totally blemish-free
pieces are reserved for contact internegs or interpositives, masking textureless areas,etc, and the leftovers for more casual use, including alteration with pencil or spotting pen, creosin dye etc. 5-mil
is a lot tougher than 3-mil and less likely to kink. I'd avoid frosted acetate - it is fragile and has a
bit of a pattern to it which might reproduce in high-contrast work.