Polyurethane Foam...Where to find?

Have you tried any big "craft" store like Michaels or Hobby Lobby?  Might even contact Home Depot, Lowes or outlets along those lines.

You want the blue stuff. It's hard to find, I don't see it at any of the big box outlets.

It's also really expensive about $ 100 for a 24" x 82" x 2" thick sheet.

Whenever I see a chunk on a construction site or in a dumpster I grab it.

A real nice substitute is a product called Balsa Foam. It is available online from Blick's. It costs about $ 12.50 for 9" x 12" x 1" boards. Comes in all kinds of thicknesses.

Obviously thats a lot more per square foot, but it's also possible to buy less.

Recommended.

The pink XPS available in most home stores is rated to 3200 psi.

But, it really needs to be wire cut for best texture (which will still want sealing.

If the polyurethane foam is the same as the foam board insulation. ...my local home Depot actually sells a one inch thick,  either 3x3 or 4x4 square for 6 bucks.  That's much more manageable than a 4x8 foot board....