Most auto glass companies can provide one, just remember that the 67-69 valiant 2 door and 67-73 valiant 4 door use the same piece. That info might make it easier to find one. These guys have them in the $325 range.
Let me know if you need a new gasket.
When I was buying one for my dart, I accidentally bought a gasket for a 2-door Post (sedan). Of course it didnt fit my hardtop, so had to buy the correct one.