Glass and mirror shops are you're best bet; but there's still no need to go there, just call in the size you need and most of the time the wait will only be a day or two.
Not sure of anywhere that would just have large sheets laying around waiting to be cut, if that's what's you're looking for.
Home Depot here has plexiglass that's available to be cut at the store. They also have windows and other glass, although I don't know what kind as I didn't really look. Try your local hardware store, you might get lucky and find what you want.