Quoted from Jeffoto:
anyone got an extra back glass?
Sorry to hear that. This is probably a very tough glass to find.
I would try the following:
1) Pinball Resource (They have a lot of Gottlieb parts, not sure if they have backglasses)
2) Backglass Restore (They would have to have a scan on file since your glass is broke)
Good luck, I'll keep my eyes out as well!