The window I used came from Frank Bear:
http://teardroptrailerparts.com/
One thing I will mention is that Fred does not include screws with the window. You'll have to make a trip to a hardware store for the proper screws. And it's about a 2 week wait if you have to order an "other than standard" trim ring...
Here's a link that I referenced when doing the install:
http://www.motionwindows.com/installation-guide.php
There are also several threads on this site with good information regarding window installation. Check out Prem's "A 12 Ft Featherlite Conversion" thread, page 26. About half way down the page he tells you "HOW I INSTALL RADIUSED CORNER RV WINDOWS IN A CARGO TRAILER." There are several other threads on window installation as well...
Good luck with the installation. Just remember to measure twice (or three times) and cut once. I won't tell you how I know...
Jack