There are many topics explaning what you can do with foamies, and are quiet a few tutorials on how you can make these things with foamies. Hell Ive made some shoulder bells, a energy back pack, shurikan and gaunts out of foamies.
Basically they are these foam sheets, not very thick, some come with sticky backing, some dont. Some come in rolls, and others come in well sheets. They all come in different sizes and colors and are VERY cheap. Foamies are very easy to work with, and I use them a lot for some of my armor pieces. You could get them at your local hobby store. If you dont know were to find them just ask sombody and they could point you out to them. They are also called "Fun Foam".
Take a look around here, you can find a whole bunch of stuff on here about foamies
It's also genericly know as Craft Foam. Should be easy to find at a crafts store or educational supply store. Just ask, if they don't know what you're talking about , they need to get a new job. It's usually in the same area as felt. The biggest draw back with foamies is they are flat, and they are thin. Perfect for doing anything flat but you got to be a little creative with getting curves with it. But it is a budget costumers dream.