The window can come from anywhere except a new one from a dealer, they are going to want close to $800 for one. At least all of them that I could find. I got on CL, facebook marketplace, the recycler and looked for cheap shells that still had a back window. I took a cardboard template of what the shape of the shell opening was, and compared that to the window for sale. Some are too big, some not tall enough etc. I know the one I have now is from a shell that fit a 97 chevy full size pickup. I even tried to find RV window manufacturers, all that stuff, but in the end, just keep your eyes on the cheap ads and go see what you can find that will fit. Of the three of these I did, two were out of people selling shells. I would usually go look at the window, and then offer them half of what they were asking for the shell for just the window. They both took it, so I paid less than $150 for both of those. The one I just did, I actually got at a secondhand shell dealer just down the street. I went in to get some weatherstrip, and just asked the guy if he had any rear windows for sale. They had about 7 behind the building, not sure why, but they were all looked like they were taken off new shells, had the exterior and interior frames and all the screws, everything there. I would have paid the guy 300 for that window, it fit perfect and i barely had to change the shape of the opening. He told me $70, and I walked out with it right then...He said he would probably never sell another one, he didn't know why he kept them. So if you have a used dealer around, you may want to try there. To me, the less cutting of the original shell, the better.
I don't know about the full convertible shell frames...I'm sure there's a metal frame in there somewhere, but try to get the mounting screws/bolts for the window to go through the metal. When it comes time to cut the fiberglass, I used a angle grinder with a masonry blade...I tried a few things, that seemed to work the best for me, plus if you hit the metal, you'll know and may need to switch off to a diamond blade if you're going to keep cutting.
If you're getting a full hardtop, it should have had a fold up window to start with, so I would try to see if you can use the same hinge mounting points for at least a couple of the bolts. Just start looking around for shells with windows, and have your cardboard template behind the seat so you can compare the shape to what you need...you'll find something pretty quick. I know the full size trucks from the mid 80's to late 90's, both ford and GM, seemed to all be about right, it's just the angles on the corners and the height of the window that you need to make sure you don't have too much cutting to do to install it.
Here's pics:
