Mega stones! You want 'em! We know where to find 'em!
First, keep in mind that you won't be able to find these Mega Stones until you beat the game and complete a slew of quests to unlock this ability. You can find the tutorial on how to start hunting for Mega Stones here.
You'll need to Fly to Camphrier Town and enter Shabboneau Castle. Head left and take the staircase to the second floor. Immediately to the north of the staircase on the second floor will be the Mega Stone for Mawile.
As always, remember to do this after 8PM and before 9PM and after touching the Sundial in Anistar City.