You can get the foam from Home Depot or Lowes in their flooring departments (at least that's where I got mine). They should sell it in packs of 4. For an interesting idea (if you haven't already thoug ...
I got myself a cheap DDR Pad, and right now I'm working on modding the thing. I found plastic vinyl covers at the fabric section in Wal-Mart and got a yard of that.
I'm hearing a lot of good things about DDR and i want to buy it. My problem is the only place i can play it is on the 2nd floor of my house. All the jumping around on the pad could make a lot of vibra ...