Mega stones! You want 'em! We know where to find 'em!
First, keep in mind that you won't be able to find these Mega Stones until you beat the game and complete a slew of quests to unlock this ability. You can find the tutorial on how to start hunting for Mega Stones here.
Extemely easy to obtain with barely any effort on your part whatsoever. Start out by using Fly and head on over to Lavarre City. Then head West of the Pokemon Center, and then directly North across the bridge. You'll notice a small well on the left. There you'll find your very own Medichanite.
Keep in mind, Medichanite will only appear if you've touched the Sundial in Anistar City and are looking for it between 8PM and 9PM.