I've seen lots of different media in the back of varying thicknesses, i don't know if they have styrofoam but they have other substrates you can carve If you're local I would just go in person as they have really knowledgeable staff and can get you set up
I have seen that in the store, best bet is to email them.
If you go to any construction site, like the one being done for Citi Group you can probably get it for free as they throw it out on a regular basis
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