It could be a long wait for a used window to fit your vintage model. It would mean that someone else is dismantling a trailer that is a very close match to yours, which doesn't happy very frequently. And if they're not local to you, there would be shipping fees.
We needed a replacement for one of the curved front windows on our '71 Tradewind and bought it through Inland RV. Models change enough that we needed to do some slight modifications to our trailer's window frame so that our model specific replacement window would fit without gaps.
Consider buying a new window, unless you don't mind a long, long wait.