At $5.94 for a 12ct package of Fibre Craft Foam Sheets, 11″ x 17″, Walmart still appears to have the best price for craft foam but in my search for costuming supplies I stumbled over Rubberflooringinc.com. They are currently selling EVA foam tiles to the tune of .96¢ a square foot. With free shipping this makes Rubberflooringinc.com one of the cheapest options I could find from a reputable dealer for small batch orders.
For those that don’t know, EVA is an excellent option for crafting armor, weapons, and other costuming paraphernalia. I’ve used it for my own amateur attempt at the Batman armor to surprisingly good effect. It’s pretty forgiving and generally easy to work with. Check out the Hard Scrabble Farm for more on working with foam.