Where would I buy this item?

I need a big block of styrofoam 12"X12"X12" for a history project. I'm going to build a Sumerian Ziggurat, but i can't seem to find a store that sells them big enough. If anyone could help me that would be great, or if anyone has any other ideas on how to construct a Sumerian Ziggurat in an easier way please do tell me. Also, if you could include the price of the materials (preferably the styrofoam block) that also would be helpful. A website would be nice to but I'd need a store website so I could go and see for myself and buy it there, rather than over the internet. Thank You to whoever helps me out :)