Use craigslist. Just make sure you inspect the tank fully, ask when the last time the tank held water, and when you get the tank home, fill it up outside or in your garage and let it sit for a day or three.
that way you will reduce the odds of getting burned.
If you are positive that you want to avoid CL, alot of petstores will sell tanks for "1 dollar per gallon" but this typically only includes tanks 55g or smaller.
hope this helps