Rhoda, I have no idea but I think I'd try first by contacting Keystone Customer Service. They might be able to point you in the right direction or maybe have one they could send you. I don't have their number at hand but it should be found on
www.keystonerv.com in the Montana section.
Also, if you have any paperwork telling you who manufactured the window and frame or if there's a sticker on the window with that info, you might try googling them and calling them.
Sorry I can't point you to a definite source. Good luck and please let us know how this turns out.