I have bought them from Precision and Steele Rubber Products. Both are too small, check Precisions website for an installation video. Either way the buffoon I had install it couldn't figure it out and kept trying to tell me the opening was too big, What a freekin' idiot for someone that has been doing auto glass work for 30 years. Anyway he ended up cutting the seal at the bottom center and filling the 1.5" gap with urethane. I still have two Steele seals I'm going to use for my 74 and my 93 I just need to call the guy that actually knows what the hell he is doing. Bottom line is they are all too small and it takes special care and sealer around the lip and glass to get them to seal correctly. They still don't look original in the corners, especially at the bottom. So if anyone knows of a better fitting seal I am all ears.
EDIT Find a good glass guy, not a Safelite kid but someone that does hot rods and custom cars as well as exotics, Those are the guys you need.
So true. The glass guy that did mine was here 4 hours and got it done but told me never again on an old vehicle. Did the windshield, quarter glass, and rear window. He was pissed when he he left. Hard to find glass guys that know the old stuff.