The Pokémon Company has announced that both the Mewtwo Mega Stones (X & Y, as seen above) are available throughout March 2017 as the Mystery Gift feature. Mewtwo isn't available in Pokémon Sun & Moon, so the only option to get the legendary Pokémon in your game is to use the Pokémon Bank.
The Pokémon Company has detailed the process required to get the Mega Stones into your game:
Mewtwo Mega Stones code: M2DESCENT
Mewtwo itself won't be found in the wild in Pokémon Sun or Pokémon Moon, but you can bring it into Pokémon Bank via Poké Transporter from earlier games. Pokémon will lose their items in the process, so currently this is the only way to get Mega Mewtwo X or Mega Mewtwo Y in Pokémon Sun or Pokémon Moon.
To get Mega Mewtwo's Mega Stones:
- 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!