A good used tank should be not too hard to locate. Whatever has happened to your tank was not that typical. As I thought about it. Someone may have removed and cleaned this tank before but used something like a strong acid. Somehow the protective layer on the steel the tank is fabricated with has gotten breeched. Or the steel the tank was made from had a very poor protective coating applied at the rolling factory.

You should have a pick and pull locally. As one option. I would do a quick cleaning and then an inspection of your old tank first. Five hundred when these tanks should almost still be a dime a dozen is somewhat strange.