If your refering to my stump I used great stuff to glue the 2 inch sheets together to make the block before carving. In the pic you see like yellow lines going sideways thru the foam , thats the great stuff.
You can find a list of manufacturers here ----------------------------------------------------------------------------------------------------> http://www.epsmolders.org/2-midwest.html
Just call or email to find a distributor
I used to work for a precast concrete company that also did prefab wall panels that used eps for insulation, we would get the stuff delivered in semi's; pieces were usually 4'x4' squares with thickness from 1" to 18".
we have arts and crafts places that sell different size and shapes chunks. Don't know if they will have pieces big enough, but gluing sheets together before you shape it is a good idea.