Looking at Wikipedia, they mention that there are indications that Mithril was present in both Numenor and Aman in smaller quantities than in Moria. I wish that the reference would have been noted in the article, as I'm wondering if it was deduced by the building of a certain aforementioned ship.
Another interesting tidbit was that Moria might have been the only soure in the Third Age, which agrees nicely with
The Might's thoughts.
Wikipedia's article on Mithril