The official Pokémon website has announced that Ampharosite, Altarianite, Latiosite and Latiasite Mega Stones are now available to players through the universal code: DRACHE. Players of Pokémon Sun or Pokémon Moon can get these Mega Stones from September 8th, 2017 until October 31st, 2017.

Get Mega Stones to Power Up Your Pokémon!

This is it! The final batch of special Mega Stones that will grant Mega Evolution powers to some of your Pokémon. This month's special code grants you Ampharosite, Altarianite, Latiosite, and Latiasite in your copy of Pokémon Sun or Pokémon Moon. These Mega Stones allow you to Mega Evolve Ampharos, Altaria, Latios, and Latias (respectively) during battle, giving them the ability to unleash awesome attacks!

Mega Stones special code: DRACHE

These Pokémon aren't available in the wild in Pokémon Sun or Pokémon Moon, but you can bring them into Pokémon Bank via PokéTransporter 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 delivery person in any Pokémon Center to pick up your Mega Stones.
7. Be sure to save your game!

Release the full strength of these powerful Pokémon when you Mega Evolve in Pokémon Sun and Pokémon Moon with these awesome Mega Stones.

Also, make sure you enter this password by October 31, 2017. That's the final day for Trainers to claim their special Mega Stones, so don't miss out on this opportunity to unlock your Pokémon's Mega Evolutions!

Source: https://www.pokemon.com/us/pokemon-news/get-mega-stones-to-power-up-your-pokemon/