Here are some locations for specific Pokemon:
Abomasite
This is given to you when you save Abomasnow in Frost Cavern.
Absolite
You will get this stone when you defeat your rival in Kiloud City in the postgame.
Aerodactylite
This one is in Ambrette Town, where you'll get it from a scientist in the Fossil Lab. The scientist is located on the right side of the lab.
Aggronite
This one is found at night and is exclusive to Pokemon Y. When you enter Cyllage Gym, go to the right and climb the first wall, then climb the next wall on the right. It is on the ground around the corner.
Alakazite
This one is found at night. Find this Mega Stone on B3F of the Reflecting Cave in the postgame.
Ampharosite
Go in Azure Bay and you'll find an old man on an island who will give this to you. The man is standing next to a fisherman and the island is just south of the Sea Spirits Den entrance.
Banettite
Find it in the Chamber of Emptiness at night.
Blastoisenite
When you get to choose one of the former starter Pokemon from Professor Sycamore, pick Squirtle and you'll get this Mega Stone. If you didn't get it from him, you will be able to buy it from a man in the Stone Emporium in Lumiose City in the postgame. It costs 150,000 and is only available every three days.
Blazikenite
You get this from the event Torchic. You need to go to the main menu and choose Mystery Gift. Select Receive Gift, then Via Internet and you'll get the Torchic after visiting talking to the woman to the left of the desk in any Pokemon Center. It's holding the Blazikenite.
Charizardite X/Y
When you get to choose one of the former starter Pokemon from Professor Sycamore, pick Charmander and you'll get this Mega Stone. Depending on the version of the game you have, you'll receive either the X or Y version of the stone. If you didn't get it from him, you will be able to buy it from a man in the Stone Emporium in Lumiose City in the postgame. It costs 150,000 and is only available every three days.
Garchompite
This one is found at night. You'll find this one near a small tower near the end of Victory Road. The tower is behind a wall that needs to be destroyed by Rock Smash.
Gardevoirite
Find Diantha in Lumiose City, who is in a small cafe (Cafe Soleil). Trade any Pokemon with her to get a Ralts with the stone.
Gengarite
A black-haired woman will give you this in Lavarre City.
Gyaradosite
You can find this one at night. It's in the southeastern section of Couriway Town. There are three small waterfalls and this stone is in the middle one.
Heracronite
This one is found at night and is exclusive to Pokemon Y. It's found in some tall grass in Santalune Forest, near the Route 2 exit/entrance.
Houndoominite
This stone is found at night and is exclusive to Pokemon Y. In Route 16, there is a spot where two skaters are going around a rock formation. The stone is on the ground on the southern side.
Kangaskhanite
This one is found at night. Go inside Glittering Cave and take the first right. The stone will be found at the dead end.
Lucarionite
After defeating Korrina at the Tower of Mastery, she'll give you this.
Manectite
This stone is found at night and is exclusive to Pokemon X. In Route 16, there is a spot where two skaters are going around a rock formation. The stone is on the ground on the southern side.
Mawilite
This one is found at night. When you enter Shabboneau Castle, go to the left and use the stairs to get to the second floor. The stone is on the ground just north of the western staircase.
Medicahnite
This one is found at night. It can be found in Lavarre City, by a well.
Mewtwonite X/Y
The stone you receive depends on the version of the game you have. You'll get the stone when you capture Mewtwo in the cave west of Pokemon Village.
Pinsirite
This one is found at night and is exclusive to Pokemon X. It's found in some tall grass in Santalune Forest, near the Route 2 exit/entrance.
Scizorite
This one is found at night. It can be found behind the Abomasnow in Frost Cavern.
Tyranitarite
This one is found at night and is exclusive to Pokemon X. When you enter Cyllage Gym, go to the right and climb the first wall, then climb the next wall on the right. It is on the ground around the corner.
Venusaurite
When you get to choose one of the former starter Pokemon from Professor Sycamore, pick Ivysaur and you'll get this Mega Stone. If you didn't get it from him, you will be able to buy it from a man in the Stone Emporium in Lumiose City in the postgame. It costs 300,000 and is only available
