Ditto is only found in Unova in the Giant Chasm area, and is one of the rare examples of a Pokémon available as catchable in every single generation without fail. Giant Chasm is on Route 13 (to the north, there's a grass area outside with identical Pokémon to Route 13), and since the road to Route 11 is blocked before you defeat the E4 (Marvellous Bridge is coincidentally under inspection at the same time, blocking Route 15), you can't get them until post-E4 (Ghetsis).
As another note, they are Lv.53,55 or Lv.63,65 depending on which kind of grass you find them in. Be sure to have a leading Pokémon that is normally easy to catch; I'm told that a Ditto becoming that Pokémon (it uses Transform and copies all of your moves with 5 PP for each move) makes itself easier to catch, too.
For more on Ditto, see here. Ditto is also the only Pokémon that can breed with Manaphy, producing Phione.