Sadly the days where things like replacement cages were readily available are behind us. You might find them, but the price might be prohibitive compared to replacing the RD. Before you buy, try straightening the damaged parts (unless they're carbon). It relatively if you have good hands. Remove and disassemble the cage and straighten the inner and outer plates as best you can, clean up the pulleys with a file or emery board then reassemble onto the RD. After remounting you might have to tweak the cage as a unit so it hangs vertically.

You may or may not get it good enough depending on the extent of the damage and you're hand skill, but you have nothing to lose by trying except a few minutes of your time.