Hi there,
I realise this thread is a little old, but I wanted to reccomend Home Depot. They sell a bag of styrefoam in their insulation section (it is not insulation though). The bag is about 10 bucks, and theres a think 5 or 6 sheets of styrefoam that are 4 feet long, 1 foot wide and about 1/2 to 3/4 of an inch thick. I bought one of those bags, and it was enough styrefoam to comepletely cover all there walls of my 4x2x2 viv with doubled up sheets, as well as build 2 basking platforms, complete rock walls and a whole cave system, and I've still got some left over. I've check out all the other stores like Michael's and other craft stores to see if I could find it cheaper, but this has by far been the best amount for price.
Good luck.