You may or may not have realized this, but there are way more Pokemon that are able to Mega Evolve than your basic starters and Mewtwo. Seriously, there is a bunch of them! The one caveat to obtaining them all is that you have to beat the game first. Keep in mind that there are a few you get just by progressing through the story.
First and foremost, you have to beat the Elite Four, which will make you Pokemon League Champion and give you access to Kiloude City, via the train system from Lumiose City.
Kiloude City is home to cool things like the Friend Safari. However, in order to progress and start the hunt for Mega Stones, you have to participate in at least one battle in the Battle Maison, located in the center of Kiloude. Once done, head all the way north to meet your rival. After beating him/her, you'll get the Absolite as well as a message to head over to Anistar City to the Sundial.
Professor Sycamore will upgrade your Mega Ring and let you know that Mega Stones only appear during the night, starting at 8PM and ending at 9PM.
You do have to fly into Anistar City and interact with the Sundial at 8PM whenever you want to go looking for Mega Stones each day.
