In addition to EVA foam (and check out Amazon too, you can find good deals since the color doesn't matter) you may also want to look into 2 mm craft foam. Joann's sells a roll for about $9 and I use it for finer detail work and smoothing surfaces, having picked up that tip from RoxyRoo and LilTyrant. Also, I cannot stress this enough, either be prepared to continuously sharpen a box cutter for your cutting or stock up on #11 blades for an Exacto or similar hobby knife. The foam dulls blades incredibly fast. Finally, check out HaloGoddess' tutorial on scaling
http://www.405th.com/showthread.php?t=31627, if you haven't already. I found it extremely helpful.