Actually, this is a very cheap job to repair. Chances are the cooling lines that attach to the radiator are leaking. Honestly, the trans if fine. Check the cooling lines to the radiator that come from the trans. Some have rubber hose that clamp on others have a bolted unit, and some have a quick connect. Check what you have and let me know.
NOTE: If the trans is shifting fine, just make sure to keep it full. It should be a cheap repair.