It is the kind of foam that fabric stores carry in about 30"x6' sheets. It is medium to high density and they should have a corner somewhere where they will have a large stack of different thicknesses.
This is a strategy I stumbled upon myself, and I have one further idea...cut NONE of the holes all the way through, and store "standing" figures base-side-up. That way, you need not grab the painted body when you pull 'em out, just the base.
Foam called plastazote is good too, it's a bit softer so can be cut to hug the base. If anyone is lucky enough to work near a laser cutter, it cuts beautifully!
Foam called plastazote is good too, it's a bit softer so can be cut to hug the base. If anyone is lucky enough to work near a laser cutter, it cuts beautifully!