Speckledhen,
Re-use-it center is a very large business that takes all sorts of used house supplies. People may be remodeling a kitchen, bathroom, porch,etc.. or even tearing down a house and want to get rid of the stuff in the house. It could even be woodwork, woodflooring, sinks, toilets, and yes, even windows. Almost anything.
You could keep you're eyes open for large dumpsters in you're area. My friend and I fixed up an old delapitated boat house for free, by dumpster diving. We got flooring, siding, roofing, wood for the walls, and yes, a perfectly good roll-open window, with a screen. The place doesn't look the same. It's beautiful. The only thing she paid for, was a couple rolls of insulation. My husband built nest boxes out of the scrap wood we found. We also found a 3 foot railing that we made aroost out of. We added hinges and and attached one end to the wall and the side that leans on the floor can be lifted up to clean under it. I will have to take some pictures one of these days and post them.
sandyj