Here is a link for possible items held by wild Pokemon. As you can see, Meditite and Medicham are number 307 and 308. Based on this chart, Medicham and Meditite never hold the Coba Berry as an item in Gen 7.
According to this article on the Coba Berry, it can only be found in a Berry pile in northeast Poni Wilds.
EDIT: In the first comment below, it said not to trust Bulbapedia as of yet, since the games are still fairly new. With that in mind, I suggest trying your luck with the tree in Poni Wilds that I mentioned above.
SECOND EDIT: Just found this out, but it is correct that Medicham may drop a Coba Berry after battling it in Ultra Space. This is disgustingly late, but just thought I should put this on here for sake of accuracy.