Try to buy new if you can so you avoid potential diseases/etc that another animal who previously used it could have left behind.
You can get individual shelves at a lot of wholesale places that shelters buy cages from/etc. I'm not sure who my shelter uses for stuff like that -but you could ask your local rescue where they get their cat shelves/etc from!
Also - i've found that ferret hammock's work EXTREMLY well in small cat cages- especially for my foster kittens- it addes more room/space to the cage and they just love it. Is this cage going to be used for a ferret or a cat? If it's for a ferret- try buying a few burlap bags to put in the bottom of the cage- my sister has two ferrets and they LOVE those!!! She does not need a shelf in her huge cage for them because she has several different hammocks and they love hoping from one to the next!
Also you can find some great corner litter pans with high backs at your local pet stores too! They also make some interactive ferret safe toys that you can hang from the cage bars.
If you or someone you know doesn't mind building a cage, i'd be happy to pass along the instructions/ lay out for the cage Colin recently made for my rabbit, Sophie. It's great!! It's extremly spacious, on wheels so it is easy to move around and clean behind, and it has two stories.