I'm using a sponge that's intended to go with a wet palette. You get it at an art store, in the painting supplies. Buy just the refills, not the whole wet-palette system (unless you paint with acrylics and want to try it). It comes as a thin sheet, about 1/16 of an inch. When you get it wet, it expands to about 1/4 inch. I don't think it shrinks back down again when it dries, like other cellulose sponges. I've cut out a piece for my kit that when folded in half fits in a Ziploc "Snack" sized bag.