Not only that, it's not just glass. Window and frame, and studs to mount it into the cab structure, are all integrated. So even if it were plumb flat, a glass shop could not just make an opening version of it from scratch.
Note to the OP - when you install that glass, put some loctite on the studs. And if you're so inclined, yank the trim on the other side and do that window as well. Torque is 22 INCH-pounds, so make sure you have a good VERY LOW RANGE torque wrench. Studs were not originally loctite-ed, and at such low torque, they're infamous for the nuts working loose. Dunno if any have fallen out, but they work loose enough that rain water can get in between the wall layers of the cab and settle at the bottom. This is a major reason why EC trucks rust back there; it's the salt spray on the outside teaming up with the pooled water on the inside. We discovered this with a garden hose test, and when I took the interior trim off and inspected, all the nuts on both sides were finger-loose.