Where to get the mega stones

Charizardite Y: Can be acquired by beating Ash for the first time outside of Nimbasa City

Charizardite X: Can be acquired by beating Alain in the Elite Four Pokecenter

Garchompite: Can be obtained by beating Cynthia after leaving the Giant Chasm. You can find her near the exit of the Giant Chasm

Houndoomite, Blazikenite, Pidgeotite: Can be found if you fly to Undella City, then go down and to the right. You will need surf and dive. Now, surf right and you will find a little island with a trainer on it. Keep going right and you will see a dark spot. dive there and you will find a underwater ruin, and there you will find them.

Lucarionite: Can be found in the Virbank Complex near the entry to the right of the main path, by taking a narrow passageway besides the Mandatory Side-looking trainer by using the Dowsing Machine near the electrical converter with the scientists.

Sceptilite: Can be found in Lostlorn forest by defeating Brendan near the top right of the area

Ampharosite: Is given to you by an NPC in the house first to the right, down from the Stadium Avenue in Nimbasa City on the top floor.