Craigslist. Just be careful and shop like you would in a store - some folks will post things like it's a great deal, and you'll find it newer later for cheaper. Most folks will give you a pretty good deal. I mean, I got a 20 high with all the trimmings plus the fish (and I could have had the stand if I had wanted it) all for $50. I got a fully set up 50 breeder for $150, with versa-top, 2 lights, 2 filters, Heater and stand, and a 5.5 and a 2.5 thrown in for free. Saw a 120 gallon for $100 the other day (If I had the cash I would have been on that like stink on manure).
Just be patient and check frequently and something good will come along.
If you don't have Craigslist, then tag sales, garage sales, estate sales, Freecycle, and that sort of thing. See if the aquarium club in your area has a bulletin board or periodic auctions.