This may not be helpful, as it sounds like you want to replace the window yourself, which is most economical, but when we had our '87 13' Burro we took it to a window place and had them replace the windows. It was a local business we knew well who had done car replacement windows for us. They did an excellent job and we never had a window problem the 12 yrs. we owned the trailer, and the windows looked exactly like the original ones.
We chose to replace the plastic windows with glass, it made for a better quality fit, the sliders worked better, it looked better, etc. The added weight
was insignificant to us with such a lightweight trailer. That was our choice, they could have replaced them with the plastic.
It was the only custom work we had someone else do, but given the difficulty we had with the windows even after redoing all the caulking, etc. ourselves it was worth it.
I imagine you could have someone custom make the windows for you and put them in yourself, saving money on labor.