I found some great dual pane windows on e-bay for my bus. These came with smoked glass and sliding screens. The vendor is Callen Campers in El Cajon, CA. http://www.CallenCamper.com
. Check e-bay for window pics and pricing.
I paid $49 for the 22" x 46". The 22" x 72" were a little more through e-bay, but I was able to get them for $49 also by picking them up. IIRC, shipping is $25. I picked them up since I was fortunate enought to live about 20 miles from them. They can order interior trim rings to fit your wall thickness.
I wouldn't say that the installation was easy, but it could have been a lot worse. The height was perfect for the original openings in my '72 Superior Coach. All I had to do was cut out the vertical beam and weld in a new 1" x 2" post. Once the sheet metal was done, I applied putty tape to the flanges and attached the window unit to the posts with self tapping sheet metal screws.
Custom windows would be easier, but cost considerably more.
Here is a pic of the Phool Bus with the windows that I purchased from Callen Campers