Yeah, I think a trip to the local school district bus lot would be in order, too. I can't imagine that type of seal just disappeared. I bet with a little looking I can find it on a newer bus, too.
As for glass installers, I hadn't thought of that. I've been working from the other direction talking to paint and body places for the metal side of things
Anyway, i also found that it's the exact same seal that is used on the rear windows, so time to call the glass guys, I s'pose...