Yes you could use a pice of plexi-glass until you can locate glass or sliding rear window. It really doesn't hold up long term, since it is easily scratched. A glass shop should be able to cut you out a piece, or check w/some of your local hardware stores, if they don't supply it they might be able to order you a piece.
Have you checked out any of the camper shell dealers in your area, they often have oem glass available for under $10.
You might also try craigslist.com in and around your area.
