I never did find anything for a seal. Although I'm sure something can be found, somewhere. It may take time to find the correct one that will work for the specific application. Surely there's something out there.
If you could provide a good picture of the end of the seal, (side view) that would really help right now.
Go to the below link. Click on the left "Universal Parts" for a seal that will work for you. I can't tell you exactly what will work. If I had a piece of it in my hands with your window out and in front of me, it would be simple.
Metro Moulded Parts Inc. Automotive Weatherstrip and Rubber Parts
You'll have to take measurements;
thickness of back door where seal slips over.
Depth of the recess (glass-to-seal)
Height of lip on window opening (to determine there will be enough rubber support to hold glass correctly)