Okay, just an idea I've had, basically a list of all megastones locations. I'll list the ones I have and where to find them, and if people post more I can add them to the list. Please put locations in small font, so if people don't want to see that one it doesn't spoil it for them!
Okay, so:
Charizardite/Blastoisite/Venusaurite: Gift from Sycamore, or if not chosen sold by a guy in the Stone Emporium in Lumiose for a varying price.
Lucarionite: Gift from 3rd gym leader alongside a Lucario.
Abomasite: Gift from abomasnow in front cavern
Aerodactylite: Gift from guy in fossil lab
Blazikenite: Event downloadable from mystery gift
Alakazite: Found in Reflecting cave on B3F. Apparently only accessible post game
Gengarite: Given to you in Laverre City
Kangaskhanite: Glittering cave post game
Pinsirite: Santalune forest in post game -NOTE X ONLY-
Heracrossite: Santalune forest post game -NOTE Y ONLY-
Gyradosite: Couriway town in post game
Mewtwoite X and Y: Pokemon villages Unknown dungeon
Ampharosite: Given to you in Azure Bay
Scizorite: Frozen Cavern behind Abomasnow in post game
Manetricite: Route 16 post game -NOTE X ONLY-
Houndoomite: Route 16 post game -NOTE Y ONLY-
Tyranitarite: Cyllage Gym -NOTE X ONLY-
Aggronite: Cyllage Gym -NOTE Y ONLY-
Gardevoirite: Traded to you in the post game on a Ralts in Luminose cities Southern Boulevard Cafe -NOTE Wanted pokemon can be anything!-
Mawilite: Shabboneau Castle
Medichamite: Laverre City Postgame
My friend helped me put this list together so thank him later!
Alakazamite: Found in Reflecting cave on B3F. Apparently only accessible post game
Gengarite: Given to you in Laverre City
Kangaskhanite: Glittering cave post game
Pinsirite: Santalune forest in post game -NOTE X ONLY-
Heracrossite: Santalune forest post game -NOTE Y ONLY-
Gyradosite: Couriway town in post game
Mewtwoite X and Y: Pokemon villages Unknown dungeon
Ampharosite: Given to you in Azure Bay
Scizorite: Frozen Cavern behind Abomasnow in post game
Manetricite: Route 16 post game -NOTE X ONLY-
Houndoomite: Route 16 post game -NOTE Y ONLY-
Tyranite: Cyllage Gym -NOTE X ONLY-
Aggronite: Cyllage Gym -NOTE Y ONLY-
Gardevoirite: Traded to you in the post game on a Ralts in Luminose cities Southern Boulevard Cafe -NOTE Wanted pokemon can be anything!-
Mawilite: Shabboneau Castle
Medichamite: Laverre City Postgame
My friend has yet to find but has noted the existance of, through internet sources, Banette's, Absol's and Garchomp's mega stone. Also he only told me the locations of the stones and not their actual names so I just wrote what I thought they were.
Oh, wow! I'll add them to the list!