Sears sells the Best Step in interlocking sheets as well, if you can find them then go for that. Flattening out the rolls is a pain in the ass. In my experience, the 1/2" foam is honestly too thick and too stiff to be any use for anything except N7 style builds.
FoamTiles.com sells 1/2" sheets but the ones I got from them were just a hair thinner (8mm), and they are much less stiff than the 1/2" ones that Lowes or Home Depot carries. Their prices are cheaper and they have free shipping.
I've also been hearing that Big Lots sells large 1/4" rolls of anti-fatigue mat for very cheap. I haven't used it personally but a friend of mine used it on her Iron Man build and the result was amazing.