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.
The Gyaradosite to evolve into Mega Gyarados can be found in Couriway Town.
Just head into the south part of the town and then head to the east where a man is standing in front of a small area of flowing water. In this area you can see three waterfalls, and the Gyaradosite can be found in the middle.
Though be sure to go look for it between 8PM and 9PM and after touching the Sundial.
