If you go to the fabric store (don't think there's one in your area? you're wrong. There are old women who like to quilt in every town on Earth), you'll find all kinds of foams of various densities, thickness, and colours. You can buy a sq. yard for relatively cheap, then just cut it with good scissors. And they have black!
There's also a place near where I live called the foam shop. You might see some foam there. In the back room or something. It's not really what they do, but if you get on their good side, they'll show you some foam. The people there are very nice. I get to see the foam every time I go.
And if you still can't find foam, go to your nearest couch, bread knife equipped, and execute slicing maneuvre 5b (cut it up). WARNING: couch foam may contain popcorn and/or loose change. Inventor not liable for injury incurred while performing or considering the acts disclosed herein.