Salvage building material stores often sell old windows. Antique dealers and rummage shops are other sources, but usually these windows are older and odd-sized. EBay.com stores have used windows for sale regularly as of 2015.
Habitat For Humanity Restores carry a wide selection of windows that are leftover from construction projects or have been taken out during remodeling jobs. Sometimes, returned windows are sold at major home improvement stores for a steep discount.
General contractors may have windows for sale at their offices. Salvation Army and other thrift stores occasionally have used windows.
Antique auction houses resell windows that are from older homes or that have some artistic merit to them. Windows must be appraised first to determine if they are worth reselling. Online stores on Etsy sometimes have windows for sale to use for decorative projects or in new construction.
Used windows are sometimes found at arts and craft supply houses since they are repurposed so often. Older used windows are not as insulative, so they get used for decorative purposes.
Many areas have salvaged materials stores with larger selections. It is important to be clear about a store's policy before purchasing used windows because purchasers may not be able to return them.