I have a Swampert and I would like it to mega evolve
There is going to be a Online Competition with an entry gift for the Hoenn Starter Pokemon's (Blaziken, Sceptile, and Swampert's) Mega Stones.
Source of Info
Currently, the Swampertite is unobtainable in Sun and Moon. However, it is possible that there will be an event that will allow you to obtain it in the future.