Unless you want to drive across the continent, do mention your city/area/state. Like Noodles suggested look locally... lots of people still get rid of their tanks for next to nothing.
Include how far you're willing to drive and what size you're looking for and how cheap exactly.
Who knows there might be members here willing to help you out.
In my opinion, the quality of fish tanks is the most important. You should refer to the famous brand such as Aqueon, Tetra, Marina, etc. Here is an article talking about the best 20gallon fish tanks