Try a home improvement store. The Home Depots near me have some white styrafoam that I think would work well. Also I don't think you will need holes pre cut in the foam for the pins. They should just push right in and make their own holes. Looking at one here it is "Poly Panel EPS insulation." From the picture it looks like it is used for insulation between wall joists against basement walls.
Here is some on Ebay $8.50 for 24 sheets 16" x 16" x 1/4"