It's "backglass" as opposed to "door glass" or "quarter glass" or "windshield".

The OP wanted glass and the immediate assumption was that it'd have to be from a yard. So I found NOS glass. Now there's a question about price? $200 is a couple of tanks of gas. If the only thing holding me back from completing a project was a $200 piece of glass I'd sure pay it. Ten years ago I got window glass from a '65 D100 and it was $50, used.

The Web is a powerful thing. And economics sure do change from time to time.

R.