Wow, that is unfortunate. "something happened" and I'm guessing you aren't sure what? I'd guess possibly a collision (rock, bird, etc), or some kind of lateral stress either from body twist or from extra weight above the window. That's one thing to be careful of when refactoring these buses, the load you add might put stress somewhere you didn't plan for.
I don't know they you'll find a stock "this is part number xyz-42" type of answer. However, if you go to an auto glass repair place, they can probably measure and either manufacture or order glass that will fit that spot. It looks like the seal is still good, so they can reuse that.
Good luck!
jim
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