Here is where I get mine:

http://www.achfoam.com/

Really depends on the quantity you want to buy. But if you want enough to make say 10 complete wings at a time then which represents about a $250 order then you should have no problems.

Universal Foam Products provides Expanded Polystyrene (EPS) products nationally through a network of over 100 shipping locations. EPS is available in blocks and shapes, and can be cut and molded into an infinite number of shapes and sizes. We also supply STYROFOAM Brand Insulation for the construction industry and STYROFOAM Brand Foam products for the floral, craft and special events industries with a $250.00 minimum order.

Our knowledge and resources make us a valuable asset for customers who demand competitive prices and fast delivery of EPS materials and STYROFOAM Brand Insulation and STYROFOAM Brand Foam

   I just saw this thread. My flying buddy and I have recently bought a foam cutting machine and some virgin External Insulation Finishing System - "EIFS"'- (EPS) foam- 1.O/cu.ft.. We both know nothing about the different and preferred foam choices, but have been given excellent advice by a number of good people on this forum, in particular, Phil Cartier and Chris Rigotti. We located a supplier in Gonzales, La. They sell different types of foam, but their main seller is white EIFS, which their salesmen said they sold to contractors for residential and commercial bldg. applications. We bought 2" and 3" thick 24" x 48" sheets. The stuff came in (6) sheets per bag of each thickness. We bought (1) bag of each thickness (2) bags total for $65.00 inc. inc. all taxes. I believe they also have 4" and 5" sheets . The company was "Building Specialty Corp., Gonzales, La. I don't know if they ship orders, but I can contact them and find out the name of the Mfg. of their foam to get phone nos. so you can find a supplier in your area.
       Cheers,
                 Doug