Phosphoric acid is sold at ACE Hardware in two forms:
Try a qt. I'm sure you will love it. The gallon size is a better deal.
(2) MUST FOR RUST, RUST REMOVER.
Phosphoric acid is the active ingredient in Navel Jelly and many rust removers. A weaker solution is found in CLR. With all acids you should wear goggles, gloves, and have a garden hose close by.
It's important you keep the part wet with this acid. Agitate it often. It will NOT eat through good steel but it will remove oxide (rust). - Dave