You may have already checked, but in my area (CA) some Home Depot's stock foil faced polyurethane foam. Some also stock the pink Owens Corning Foamular polystyrene foam ... takes a bit of driving around but I can usually find it. Both are used for insullation.

I haven't sued the polyurethane stuff yet but I have used Foamular for the interior of S&G waveski's. It works easy and is great for plugs and such. The surface isn't totaly flat and requires a little sanding if ya need a flat surface. The problem I've had with it is gluing it ... the stuff isn't porus and if the glue cures by evaporation or off gassing ... the glue won't cure ... I've tried all sorts of stuff ... so I just use epoxy. You can glue it with pour foam though if precision isn't required.

For raw foam I'd stay away from pour foam unless you run out of alternatives ... expansion is a bit unruly and the higher density good stuff is pricey.

Ken Blanton