It does, and the Dwarves and Gondor both had some. Recall that the King of Men has been crowned, and doubtless gifts to him would come from far and wide. Dwarven greed doesn't extend to friendship, as we saw with Bilbo, and no doubt Gimli could arrange for some supply of Mithril to be given to the greatest of the lords of Men, his friend. It makes total sense, that, in reward for such friendship, Gimli would secure a great gift. You overestimate Dwarven greed.
As for Gandalf, he said it was found alone in Moria in the world, which could easily be referring to Middle-earth only. Numenor is not part of Middle-earth, which is the continents of Harad, Rhun, and the West. Gandalf would doubtless have knowledge of Numenor's mithril, and perhaps this was an editorial oversight--that is, Tolkien only concieved of Numenorean mithril later on and did not amend it. In any case, it is far more logical to take Gandalf's words to be true, adding in what we *know*, rather than inventing locations for Mithril.
Ithilion, Discussions Moderator(Auta i lómë) 14:19, April 22, 2017 (UTC)