I can't help directly but I do have a few ideas for you to consider.
What about second hand (i.e. used) parts? I was able to buy a used part for our Airstream trailer (granted, not an Interstate) from a salvage yard - check with
https://colawrvsalvage.com/ to see if they carry any Interstate parts. Since they're a salvage yard their inventory will change periodically so you should call them from time to time. Keep in mind the same windows were probably used on other brands so maybe the Colaw rep can point you toward something that will fit.
There are quite a few Sprinter resources and useful information on this forum
https://sprinter-source.com/forum/ Do some searching on CRLaurence to see if anyone on that forum can direct you to a source.