Its kind of like a heavy translucent ground cloth. Its rolled up and I buy at lowes where they keep trash bags. I think it is 8mil thick or so.
Just a big ol piece of translucent piece of thick plastic wrap is all it is.
Its cheap and works well to RP with.
Might be in the paint section too depending upon the store. The more diffuse and thick, the better. 12 or 16 mil sometimes. Anyway. Cheap.