That is a standard piece of glass that a regular glass company should be able to get for you. If you can't get a new one, then I suggest that you start looking at salvage yards to see if they might have that model car in their inventory. While your car is a Buick, it is the same glass that was used in the Chevrolet Caprice Classic Station Wagon. The only problem that I can see is that there are 2 different versions of that vehicle. One has the rear vent glass and the other doesn't. Just make sure that you tell them that you need the glass that have the rear vent window. Possibly one of our members that frequent "pick a part" yards might be able to find one for you.
UPDATE: I just saw the picture of the piece of glass that you are needing. From your description the rear glass would be the one that I mentioned above, however, your picture shows a door glass. This will be a lot easier to find. Go to a glass shop and bring your car and have them look up the NAGS (National Auto Glass) part number, and if it is not available, ask them what other cars also used that glass. Many times a piece of glass was used for many years on different models within the General Motors brands. Once you have the NAGS part number and a list of other cars that also used that glass, your search will be a lot easer to locate the illusive glass. You will also need to advise whether it is a clear glass or a tinted glass. If it is tinted, it should be marked as such in the lower corner of the glass as "soft ray" or some other name that GM was using at the time.