Yeah - good sources for EPS foam. There are principally two foam types that you should restrict your choice to: the EPS that Scott mentioned and Polyurethane. 2+ pound density will work just fine. PolyU comes in densities up through 12 pounds and is fairly easy to locate locally (sign manufacturers carry it). You can laminate PolyU with Polyester resin or Epoxy. The other foam is EPS Expanded PolyStyrene - what we typically call beaded white foam or styrofoam (although that is incorrect). You can ONLY laminate EPS with Epoxy because the styrenes in Polyester will melt the EPS. Like Scott mentioned, most insulation companies carry it, just be sure to ask for 2# or higher. I think that 1 and 2 pound is pretty readily available and 3 pound or higher takes some searching. Brewketle uses 3 pound foam and may have a source for you. I have had good luck with Architectural Foam places, also. They carry 1, 2 and 3# in my neck of the woods.
Stay away from XPS which is the blue and pink insulation foams found at the big box stores. Pink from Corning and Blue from Dow - Dow calls their Blue XPS Styrofoam. It's NOT the same as the White EPS mentioned above. XPS is co-polymorized with polyethelene and nothing sticks to that stuff. Very likely that it will delaminate when warmed up in the boat.
There are a number of supply stores that sell close tolerance blanks. Foam EZ and Fiberglass Supply, I think, are probably the two most popular. They both carry PolyU and EPS blanks. Although they are both out here on the West Coast and most likely would double the cost of the blank to ship it. The close tolerance blank is molded to the basic shape of a surfboard, requiring less shaping to finish...and you pay for that convience.
Foam EZ is more businesslike, Fiberglass Supply has better prices, but are WAYYYYYYYY laid back about getting your order out.
I'm not sure if a Florida supplier would be cheaper shipping, but Surf Source is a reputable supplier.
Keep us posted on your build!