This is best done in steps.
- Make sure your Mega Ring has been upgraded. If it hasn't, here's how you can:
- Go to Kiloude City and participate in a battle or two in the Battle Maison.
- If you played in the Battle Maison, your rival should be at the pond area and the top of the city. Fight him or her and win.
- Go to the Anistar City sundial. You should meet Professor Sycamore there. He'll upgrade your ring.
The Mega Ring upgrade is necessary to find the Gyaradosite and many other Mega Stones, so make sure you do it!
2. Make sure the time on your 3DS clock is 8:00 PM - 8:59 PM. If it is not, the Gyaradosite will not appear.
3. Go to Couriway Town. If you visit an area that overlooks some of the waterfalls, you should find a sparkling object. It is the Gyaradosite.
Unfortunately you can't steal Mega Stones, so this is the only way you can get one besides trading.