Here are the mega stones locations as far as I know.
EARLY ON IN GAME
Charizardite x and y from professer you get stone if u pick charmander can buy later on at stone collector in luminose city
Blastoisenite From starter or can buy from Collector
Venusaurnite From starter or can buy from collector
Lucarionite From gym leader with the 2 lucarios gives you one with stone
Abomasnowite save abomasnow and he gives you the mega stone
Aerodactylite Fossil lab a scientist gives you it
Gengarite Lavarre city a lady in a black haired lady will give you it
After you beat game defeat your rival and talk to the professor at the sundial and he upgrades your mega ring to finding megastones at night from 7-8 aclock when the sundial glows.
Heracrossite Y At the entrance of route 2 in tall grass
Aggronite Y At cyllage gym its in a corner
Tyranitarite X Same as Aggronite
Alakazamnite In the Reflecting cave on the third level is the stone only at night.
Houndoom Y At route 16 two trainers on rollerskates are skating around a Giant Stone and the megastone is at the south of it
Kanghaskanite At the Glittering cave take the first right and it is at the end of the passage
Manectite X same as houndoomites location
Pinsireite X Same as Heracrossite’s Location
Scizorite Behind the Abomasnow at Frost Cavern
Gyradosite South East of couriway Town there are three waterfalls the middle one has the stone.
if anymore are found I will add more.
I give credit to Pokemon x and y guides around the internet for the location of some of these Stones.