Try the yellow pages. You are looking for Closed Cell Polystyrene Foam also known as styrofoam, floataion foam or white foam. The pink or blue foam boards found at building suppliers (Lowes, Home Depot etc. are open cell foam (look close at the end or edge and it will look like a bunch of air bubbles. Closed cell foam (white) looks like a bunch of marshmellows squished together (cheap foam coolers) up close. Then try a local marine dealer, Dock or building suppliers (see note above), then a hobby shop that sells supplies to Radio Control Model Airplaners. They use a closed cell (white) foam for wing cores and should have a block large enough. A mail order hobby shop (Tower, Horizion, Hobby Lobby etc.) is another possibility. A quick search on the internet found American Foam Products (americanfoamproducts.com) and Drew Foam (drewfoam.com) as additional possible sources, contact them to be sure. 3M 77 Spay Adhesive (a spray contact adhesive) may be used to glue the foam slices back together. Test it to make sure it will not disolve the foam (Rumor in the model airplane circles says the newest formular does). Good Luck.
You might try an aircraft supply. I am currectly restoring a 1965 Sunfish and I opted to remove and replace the original foam blocks as they were extremely waterlogged. I got new foam from an aircraft supply warehouse for DIY homebuilt aircraft. The foam in the Sunfish is 1lb density closed cell foam which is white. Good luck.