The following is a list of Pokemon and how to obtain their Mega Evolutions. They are divided into four categories: Mega Evolutions you can obtain before completing the main story (Pre-End), those you can obtain afterwards (Post-End), ones only available at night (Only At Night), and ones that have other special requirements (Other):
Note: More Mega Stone locations coming soon!
HOW TO OBTAIN
Professor Sycamore gives you Charizardite X or Y when he gives you the Pokemon (depending on your version of the game). If you didn't get one from Professor Sycamore, you're able to purchase it from a man for 100,000 in the Stone Emporium in Lumiose City every three days (his stock changes every day) after you finish the game.
Professor Sycamore gives you Blastoisite when he gives you the Pokemon. If you didn't get one from Professor Sycamore, you're able to purchase it from a man for 500,000 in the Stone Emporium in Lumiose City every three days (his stock changes every day) after you finish the game.
Professor Sycamore gives you Ivysaurite when he gives you the Pokemon. If you didn't get Venusaurite from Professor Sycamore, you're able to purchase it from a man for 300,000 in Lumiose City (his stock changes every day) after you finish the game.
Korrina gives you Lucarioite when she gives you the Pokemon. Hey! Everyone! Free Lucario at Korrina's house!
After saving an Abomasnow in the Frost Cavern you receiven Abomasnowite.
Travel beneath the Sea Spirits Den in Azure bay, and a man on an island (next to a fishing trainer) will give you Ampharosite.
In Lavarre City a black-haired woman at the top left of the city will give you a Gengarite.
Speak to the scientist on the right side of the Fossil Lab in Ambrett town to get your shiny new Aerodactylite.
HOW TO OBTAIN
Defeat your rival in Kiloud city and you'll receive an Absolite.
Capture Mewtwo in the cave to the west of the Pokemon Village and you'll get Mewtwoite X or Mewtwoite Y (depending on your version of the game).
Trade a Pokemon (any will do) with Diantha in the cafe to the south in Lumios City where you first met her. She'll give you a Pokemon holding Gardevoirite.
Only At Night
After defeating the Elite Four and completing certain missions, you can find Mega Stones at night between 8PM and 9PM (real time) at certain locations around the world where the ground is sparkling. Interact with these spots to get the Mega Stone.
HOW TO OBTAIN
In the southeast part of Couriway Town there are a bunch of rivers near a waterfall--here, you can find the Gyradosite.
In front of the well in Laverre City, Medichamite is sparking on the ground.
Head to the Glittering Caves by way Ambrett town (after riding the Rhydorn on the path South of the town). Once inside, take the first right--Kangaskhanite is at the end of the path.
There's a spot in the Reflecting Cave where there's a glowing landing--you likely thought there'd be a Legendary Pokemon when you came here earlier. Nope! It's a Alakazite.
Head over to the Chamber of Emptiness for your Banetteite--turns out it's not so empty, is it?
Near the end of Victory Road there's a small area outside, and there's a stone tower at the top left. Use Rock Smash and walk in front of it for your Garchompite.
In version X, go to Cyllage City gym (the rock climbing one) and head to the second level to the right. After climbing, you'll find Tyranitarite.
In version X, go into Santalune Forest from Route 2. Go straight up and Pinsir is on the left in the tall grass.
Behind the Abomasnow in the Frost Cavern you'll find Scizorite. It's hard to see--but it's there.
On the second floor of Shabboneau Castle, (on the left) you can find Mawilite. Yes, it's indoors.
In version X, eave Dendemille town towards Route 15 and go south--you'll find yourself on Route 15. Follow the path until you see two skaters circling a large rock, below that rock will be Manectricite.
HOW TO OBTAIN
You can't actually get it in-game as it comes with the Torchic that comes free. On the main menu, select Mystery Gift, Receive Gift, Yes, then Get Via Internet, then Yes again. Then go into the game and talk to the woman to the left of the desk in any Pokemon Center. You'll get a Torchic and Blazikenite for when it's all growed up.