The Pidgeotite, Steelixite, Heracronite and Houndoominite Mega Stones are now available via a password unlock for Pokémon Sun and Moon. The Mega Stones password is: AZUL

Get Another Group of Mega Stones

Four more Mega Stones are available for your Pokémon Sun or Pokémon Moon game.

Add more Mega Stones to your collection so you can Mega Evolve more Pokémon in Pokémon Sun and Pokémon Moon with a special code. When you enter the following password in your game, you'll receive Pidgeotite, Steelixite, Heracronite, and Houndoominite Mega Stones, the items needed for Pidgeot, Steelix, Heracross, and Houndoom to Mega Evolve during battle.

Mega Stones password: AZUL

These Pokémon aren't available in the wild in Pokémon Sun or Pokémon Moon, but you can bring them into Pokémon Bank via Poké Transporter from earlier games. The Pokémon you transfer will lose their items in the process, however, so this distribution is currently the only way to get these Mega Stones in Pokémon Sun or Pokémon Moon.

To get the four Mega Stones:

1. Select Mystery Gift on the main menu.
2. Select Receive Gift.
3. Select Get with Code/Password, then Yes, and then Yes again to connect to the Internet.
4. Enter your password.
5. Watch as you receive the Mega Stones.
6. Speak to the deliveryman in any Pokémon Center to pick up your Mega Stones.
7. Be sure to save your game!

Enjoy bigger battles with the force of Mega Evolution in Pokémon Sun and Pokémon Moon with these awesome Mega Stones!

Source: http://www.pokemon.com/us/pokemon-news/get-another-group-of-mega-stones/