Try these guys first and see what the price is like, if to high I know lowes sells a styrofoam like (foam) sheet that works great under the tanks, it blue. Couldnt tell you who makes it.
If all fails looking in the phone book,each county has a company that manufaturers styrofoam for shipping and packing. I get mine from a company here in jersey, they will make you any size or shape, but wich is great for the 3D Backgrounds.
Specialty Products Tampa, FL
Phone: 813-621-5371
Manufactures fiberglass insulation; manufactures plastics materials & resins; fabricates pipes & fittings; manufactures plastic foam products
Products: Styrofoam; covers; covers: insulation; fibreglass materials (fiberglass materials); fittings; fittings:..
Hopefully this is some help