You can buy large sheets of "Styrofoam" (actually polystyrene foam) at home improvement places like Home Depot or Lowes, etc.
It will be sold as various kinds of insulation, and will be the denser type of ps foam (the "extruded" type rather than the "expanded" type) which will be easier to carve or shape if you want to do that --and a lot cheaper than getting large sheets of the white "bubbly" type unless you find that at a packing store.
If you want to end up with a shape that's thicker than these sheets are, just cut smaller pieces and stack them together (with adhesive), then shape that block. Lots of impressive ps foam pieces are made this way.
If you want more info on doing this, and/or on handling ps foams in general (glues, tools, websites, etc.), check out this page at my site:
http://glassattic.com/polymer/covering.htm
(...click on the subcategory called *Shaping, Cutting, Gluing* ps foams, under the main PLASTICS category)
(...also click on *#6 Polystyrene/Styrofoam/etc*
......then scroll down a few paragraphs to the subcategories on *Polystyrene Foams* and *Extruded PS Foams* if you want more info on the sheets I mentioned, their brand names, etc.)
HTH,
Diane B.