According to Serebii.net, a Mewtwo Megastone distribution is currently ongoing for Pokemon Sun & Moon!
Players will be able to receive Mewtwonite X and Mewtwonite Y with the code "M2DESCENT" at the Mystery Gift "Get with Code" section of the game.
Although Mewtwo cannot be caught in Pokemon Sun & Moon, it can still be retrieved from Pokemon Bank or traded from other players.