You can only get it post-game.
You can find it in Couriway Town.
Head into the south part of the town and then head to the east where a man is standing in front of a small area of flowing water. In this area you can see three waterfalls, and the Gyaradosite can be found in the middle.
You can only get it between 8PM and 9PM and after touching the Sundial, so make sure you do that first.
Hope I helped.