Or you can call Humphrey's and order a window but be prepared for the price sticker shock. http://buysellfind.net/siding/humphrey/concession.htm
Or for another option on service windows call Challenger Door Company in Nappanee , IN. I posted their phone number in a previous post on concession windows. Do a search here on Roadfood. Everyone else I talked to wanted $800.00 and Challenger built me a simple slider 48 X 31 two pane with no screens for $164.00 including tax. And they are a Amish company so they seem to be very careful in quality but the shipping on any window is outrageous because of the size. And they will tell you your better off if you order it and come pick it up. They are in the middle of northern Indiana. I think I'm going to take the step van up there and drop it off and return the next week and pick it up. That way they can cut the holes and install the windows. Again I'm impressed with their craftsmanship, I looked at some awesome stainless steel cabinets with glass doors they built for another customer when I was last up there. And you can't beat the price.
PS reread Wild Dogs post on his build there is a ton of very good information in that thread.