I'm wondering quite intensively...
Aerodactylite, Ampharosite, Blastoisenite, Charizardite X/Y, Venusaurite, Lucarionite, and Blazikenite are all the mega Stones available before the 5th Gym. The last one you obtain before heading to the 5th Gym is Ampharosite.
Lucario, and whichever of the 3 original starters that you chose+ blaziken if you do the wifi event
I think there may be a few more that random people give you. there are more detailed location guides out there if you search for xy megastones, but most of the megastones you have to wait until you can upgrade your ring at the end of the game.