What is styrene and where do I find it?

What is styrene and where do I find it?
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Custom WorkstationI've seen many customizers use something called styrene. My question is what is it exactly, how flexible is it, and where do I find it? I heard that it is basically a plastic sheet, but all the info I find on styrene says it's basically foam. Which can't be right, because that doesn't seem very useful for hard surfaces.
Any info would be greatly appreciated.

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Action Figure on eBay
Then they just say "yeah we don't have it."
Quite annoying.
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