I agree with marisa, I haven't read your story but will be on my way to do so. If there is a chance your missing animal is still around, its a good idea to wait. No harm in fidnijg out now though, since you still need to do a lot of research on the animal you decide on should the tank become available.
Well what are you looking for? Are you looking for a snake, lizard, amphibian? Do you want to handle your pet or just look at it? Do you want something that can stay in the 20gal its whole life or just to start? Do you want something that is relatively easy to maintain or something that is more challenging? There's an awful lot that can go in a 20 gallon... if you give us some more specifics we will be able to give you better ideas