The Pokemon Company has announced a way to bring Mewtwo's Mega Stones into Pokemon Sun and Moon. By using the code M2DESCENT in the game's Mystery Gift function, players can obtain both a Mewtwonite X and Mewtwonite Y to use in Pokemon Sun and Moon.
Mewtwo isn't actually in Pokemon Sun and Moon, but players can use the Poke Transporter to transfer the Legendary Pokemon into the new games. Because the Poke Transporter doesn't transfer items, this code is the only way players can currently obtain the mega stones for use in Pokemon Sun and Moon.
Codes for other currently unavailable Mega Stones are also on the way. Players who didn't participate in last month's International Challenge Competition will get a code for Beedrillite and Mawilite in April 2017.