I think anyone who ever did any camping BT (before Thermo-Rest) probably had a blue pad at one point or another. Mine didn't come from Wal-Mart but they were pretty much found in any self-respecting outdoor store.

I bought one last year at Sport Chalet and cut it up to make a couple of sit pads and a couple of cozies for my insulated bottle and one of my pots for winter camping. It is also good for setting a stove and fuel bottle on if the stove has feet on it. I think it was about $10.