Ditto is located in route 218.You must have the national pokedex, defeated the elite 4 and obtained the pokeradar. OK, here are the steps.

Go to route 218 (the route that is right next to Canalave City). Use the pokeradar in the grass patch closest to the lake. Look in one of the grass patches that have been shaking and one of them should soon have a ditto in them. Take your time because its very rare in this game.