I'm just going out on a limb here, so don't flame me for my suggestion, but if I were you, I'd try a gun shop.

Seriously, though, give them a try, OOOORRRRR, look in your yellow pages for a fabric store (dunno, but Joanne fabrics is in my area) and see if they have it.
You are referring to the "egg crate" type of foam, yes? If it's something else altogether, let us know.