So did I....
the stiff stuff is generally sold as anti-fatigue flooring It is closed-cell bubble but as hard as ....!!! It comes in all thicknesses....all as hard as the other. I think this is the rubber stuff.
The other stuff is the foam. I found after much searching is again sold in different thicknesses but is generally used to make wetsuits and stuff. That's the one you want.
They have hard neoprene rubber sheets there to, I have a 1/4" thick 12x12" square of that in anti-static that I set my mobo on.
I usually get mine from DD.Conformal/Grease & Neoprene ..Or on eBay I've found big sheets pretty cheap at times.