I finished the game (getting Yveltal and beating Elite4), but I went to places where mega scizor and mega garchomp stones, but they are not there. Anyone help?
Did you beat your rival in Kiloude city yet? If you haven't, that's why. After you beat your rival, they will give you a mega stone and tell you to go to anistar city. When you get there, you should see professor sycamore. After you talk to him, every day at 8-9 pm the giant crystal in anistar city will give off a light, enabling you to get the rest of the mega stones. The light will die down after 9pm. The mega stones look like little stars on the ground, instead of regular pokeballs.
Hope I helped!
Have you upgraded your Mega Ring yet? If you haven't, that's the reason.
To upgrade your Mega Ring you need to need to get to Kiloude City (If you don't know how to get there look here.) You then need to battle your rival. (Serena/Calem) Once you battle your rival, go to Anistar City and talk to Prof. Sycamore. He should upgrade your Mega Ring, which allows you to get the Mega Stones. Also note that it has to be between the times 20:00 (8:00 PM) and 21:00 (9:00 PM).
Hope this helps!