The Pokémon Company is currently giving away the Mega Evolution Stone for Mewtwo, The Mewtwonite X & Y for Pokémon Sun and Pokémon Moon.
You just need to use the code: M2DESCENT on the game’s mystery gift feature found on the main menu.
I know this may be weird especially since you can’t get Mewtwo in-game. Even the update also mentions that the only way for you to get a Mewtwo in Sun and Moon is by getting the Pokemon through the Pokémon Bank. The game’s paid storage service.
Here’s the exact process on how to get your Mewtwonite X & Y:
Select Mystery Gift on the main menu.
Select Receive Gift.
Select Get with Code/Password, then Yes, and then Yes again to connect to the Internet.
Enter your code.
Watch as you receive Mega Stones.
Speak to the deliveryman in any Pokémon Center to pick up your Mega Stones.
Be sure to save your game!
For more info and updates, you can head to the Pokémon site.