You can also check local yard sales. You can usually find listings which mention tanks. These tanks typically need some work but you can pick them up for a few dollars.
Facebook! My area has a lot of buy sell trade pages and I recently got a 55 gallon and a 29 gallon (both with stands, filters, heaters, vacuums, EVERYTHING) for $100. I was one happy lady!
Just be careful you don't get stuck with fish you don't want and that you might find difficult to rehome; a lot of tanks come with convicts, clown loach, oscars, big (or potentially big) common plecs and the like; steer clear of those!