These are few places we have used to get free glass for greenhouses. Usually its best to get your windows first, then design your greenhouse around the windows you can get. When it comes to free glass, you don’t get to pick your dimensions.
I learned that you can get cheap or free windows from window sellers from the book The Tightwad Gazette, so the information may be about a decade out of date.
Often an order of windows will be just slightly off, by maybe only an inch or two. These slightly incorrect windows will sit, perfectly good, in the glazier’s wares area and collect dust. It’s worth checking with these establishments to see if they have windows you can pick up for pennies on the dollar.
Deconstruction or Demolition sites
More popular recently than a straight “demolition” is a slower and more methodical “deconstruction” of old buildings. Windows, doors, and plumbing fixtures are carefully removed, cleaned off, and resold at drastically reduced rates. One of the places specializing in selling these goods is ReStore. Google “restore” and your city name to find one in your area.
Depending on how much an “open door policy” your dump has, you can pick up practically anything you could possibly want if you have the tenacity and patience to visit the dump regularly.
Craigslist or Freecycle
Last on the list because it’s the most obvious. This is where most people will start their search, so you may be competing with other greenhouse builders in your area for that free glass.