Any glass shop that can cut safety glass should be able to get you some. I use a coupla of tools designed for windshield glass work. With the lockstip out, I work the glass in and the outer edge of the gasket over the glass. Some I guess use a thin rope or string to flare the gasket, but on a large truck that takes two people, and I've always did mine windshield by my lonesome self!
A little rubber lube or at least some soap goes a long way in getting the glass to slide in the rubber.
The gasket material is sized by the glass thickness, the body flange thickness some have more overlap than others, but if you walk in with a piece of what you have now, they should be able to match it up.
My Dart has windshield corners that the angle is tighter than 90 deg and I was able to bend the stuff that tight.
Unlike car and light trucks in the 50's, big trucks use a common "lock strip" type gasket that is common to all kinds of industrial glass installations.
It is helpful to have someone who does glass work help or at least show you how to do your first one.
Flat glass is easier than "bent" glass to install, once you get it close they just pop the rest of the way in, then you install the lock strip to lock it in place.
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