Use RV-type windows, for the reasons cited by the others. They are readily available in most any size and type you like, and it usually doesn't cost any more to get a custom size made (there are no "standard" sizes anyway).
Here are some possible sources, in addition to Ebay and Amazon:
https://www.all-rite.com/windows
http://easyrvwindows.com/rvwindows.html
As for the leaky ones, odds are they just need to be removed and re-installed with new sealer applied. Butyl rubber sealing tape is the simplest and most effective method. It's a simple task to remove the screws from the metal frame, pry the window loose, clean and apply new seal tape.