Looking for a nano cube or similar 20g + needs to be in really good shape.
Several cubes for sale over on MichiganReefers. Prices seem high lately though. Though, they do seem to be more high end setups e.g. upgraded lights (LEDs or HQI), newer tanks, etc.
Good plan to start out 20g+...my 12g nano was a lot more difficult to maintain vs. my 24g that I ended up keeping.