An online event was announced today (June 23) on the official Pokémon website that allows players to receive several previously unattainable Mega Stones for the Pokémon Sun and Moon games. This event allows you to receive Mega Stones for the final evolutions of all three Hoenn starters, as well as a couple of others. By following the instructions below, you can get a Sceptilite, a Blazikenite, a Swampertite, a Cameruptite and a Banettite, which are the Mega Stones that allow you to Mega Evolve Sceptile, Blaziken, Swampert, Camerupt and Banette respectively.
These Mega Stones are available via Mystery Gift, so when you start up your Pokémon Sun or Moon game, select “Mystery Gift” instead of “Continue.” Next, select “Receive Gift” and then press “Get with Code/Password,” then “Yes” and then “Yes” again. Now you will have to enter your code, which for this event is “MATSUBUSA.” After your game receives the Mega Stones, you will have to personally retrieve them yourself in-game. So click “Continue” and then go to any Pokémon Center where a delivery man will be waiting with your Mega Stones.
It’s also likely that you don’t have a Sceptile, Blaziken, Swampert, Camerupt or Banette to use these Mega Stones on, since none of them appear in the wild in Sun or Moon. In order to get these Pokémon in those games, you will have to already have them in another game, likely Omega Ruby or Alpha Sapphire, and then send them over to Sun or Moon via Pokémon Bank. Any held items your Pokémon are holding will be lost in this process, including Mega Stones, so being able to Mega Evolve Sceptile, Blaziken, Swampert, Camerupt and Banette in Sun or Moon was impossible prior to this event.