You probably want extruded polystrene foam rather than expanded ps foam since it’s much better for shaping. I think you’ll have to buy sheets of extruded ps foam at a home improvement or hardware store (they’re usually various kinds of insulation) then stack as many layers as you need to get the size block you want though, as mentioned before.

The following page at my site has info about the various types of polystyrene foam that can be shaped, and how to glue and shape them in various ways, etc.:
(…under the “Plastics” category, click on the sub-category called
“CUTTING, GLUING, SHAPING” and also on the sub-category called “TYPES”)

One of the links in that section will take you to this site, which has a lot of ps foam objects to look at as well as info about the tools they sell for shaping ps foams, and probably CD’s they sell, etc.
(…that page goes to various projects, and there are lessons even in some of them)
…lots of examples and some lessons on using tools to shape various ps foams, and other info: (this is the “Foam Art” gallery page… be sure and click on the Dog photo, then see a stacked block of ps foam in the background of one of those photos)

HTH,

Diane B.

Home Depot sells large sheets, about 2 inches thick, buy about 3-4 feet. You could glue a few together to get the thickness needed.