Where can you get tempered glass?
Call around to glass shops and ask if they have any that has been abandoned by the customer they prepared it for. You might get lucky and score a sheet.
Other places that have it are construction resale shops in the form of storm doors or storm windows.
If you don't mind it having a greenish cast to it you can get it from windshield replacement shops. I've gone and asked to have what's in their shop vac but that can be a mixed blessing. If the car owner had a pet or smoked you'll be getting a lot of other stuff along with your glass.