I believe they're just ordinary plexiglass. You could probably buy and cut your own using the old as a template. It'd be a good idea to use a new gasket when you reinstall. There are some old threads with how-to help and sources for the gasket. A site search/Google (in the search menu in the blue bar under your log-in information) will help find them.
They're a bit of a pain to install (or so I've heard), so I'd want to do it right with brand new materials and be done with it for another ten or twenty years.
If you don't want to do the install yourself, an automotive glass shop would be your best bet.