Hi Spencer, if it's just the glass that's missing.. pull the window and take it to a glass shop. If the frame for the window is missing... well, you're talking about a window that hasn't been made for years and you do have a 31 year old trailer. If you can live with it not opening, the glass shop can fix you up.
Or if it's the radius sliding window,
Scamp sells them about as cheap as you'll find anywhere.
Good luck!