Around here (Wichita, Kansas) the "hobby & craft stores" (Micheals & Hobby Lobby) don't carry styrene, but the HobbyTownUSA sells Plastruct and Evergreen brands--I've bought "plain" 12" X 24" sheets there before.
We also have a model Railroad hobby store that carries both brands, but nothing larger than 6" X 12".
You can find "ghetto" styrene at Wal-Mart in the form of those thin plastic FOR SALE/GARAGE SALE/ etc signs. Old CD jewel cases are good for building too.