I agree with the Craigslist idea. I've been haunting it myself and I keep seeing lots of different tanks.
I would advise you to use the widest search parameters possible.
For instance, I have been searching both the "All Community" and the "All for sale" categories using the search words "Aquarium", "Fish" and "Tank", occasionally I've even tried "gallon" (search using those words separately, not as one search string.)
And be patient! I've been searching for the last week or two and new stuff is posted daily. Some folks want WAY too much for their used aquarium (like twice what you would pay for new) and others offer an amazing deal! (There was a 29 gallon, with everything you could possibly want except the fish and gravel, AND a 5 gallon bowfront thrown in for free, all for $150! I was SOOOO kicking myself that I don't currently have room!!!)
Well that's the thing, I have messaged at least 10 sellers of 20 gallon tanks to have all reply they sold already. One posted today I literally missed out on by 5 minutes or so, according to the seller, sold for $35 with the stand and random accesories.