How do you get the Dark Stone in Pokemon X and Y?

You can find a Leaf Stone in Route 8 and as a hidden item in the mushroom next to the Café in Laverre City.

Alternatively, it can be bought for 2,100 Pokédollars at the Stone Emporium situated at the Vernal Avenue in LumiOSe City. It's the lane right next to the Poké Center in the South Boulevard.

You may even get one in the Secret Super Trainingcourses, which need to be unlocked by having a fully EV trained Pokémon.