Look in your local Yellow Pages under "Packaging" for a company that uses foam for custom packaging. Closed cell foam is typically used as padding in packing crates and cases, but you have to be careful about the type. Specify that you're looking for "Minicel" (a.k.a., "micro-cellular polyethylene foam") and not "Ethafoam", which is a type of polyethylene foam with larger cells that's not well suited for most kayaking applications.