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.
Finding these version exclusive Mega Stones is rather easy. Simply make your way to Cyllage City Gym, or just use Fly to get to Cyllage City quick.
Upon entering the gym, take the first rock wall up to your right, climb the set of stairs and then once again climb the rock wall to the right. You'll come up to a dead end, which will have either the Tyranitarite (in Pokemon X) and Aggronite (in Pokemon Y).
As always, remember to do this after 8PM and before 9PM and after touching the Sundial in Anistar City.