Well tonight I made that particular discovery.
From this page on www.tk560.com
Sometimes called HIPs [High-Impact Polystyrene Sheet] I've had the best luck searching for HIS [High-Impact Styrene Sheet] Below are some suggested sources.
The problem that brought this up was, I'm wondering if this is a practical way to build some new trim pieces for my car. The old pieces just arent holding up. Or maybe just use the plastic to mold more durable fiberglass bits.
Of course that would require a nearly 4ft vacuum table. :)