Nobody ever buys used windows to be put in a house, so they aren’t kept around for anything useful. Some window contractors donate good usable windows to resale shops for non-profit organizations. ( Habitat for humanity, Womens protective serv..etc..) they sale the used windows (and almost everything else) to fund their charities. I would suppose the best place to start would be contacting window replacement companies. 90% of the time they are just thrown out. I don’t know why they wouldn’t let you have them as long as it didn’t require any extra effort or cost on their part. Hope this helped Good Luck!!