Where do you get an Ditto in Pokemon diamond?

You can't actually find a Ditto in Pokemon Diamond. Ditto is not available in the original Pokemon Diamond game.

However, you can find Ditto in the following games:

* Pokemon Pearl and Platinum: Found in the Grand Underground.

* Pokemon Brilliant Diamond and Shining Pearl: Found in the Grand Underground.

* Pokemon Black and White 2: Found in the Undella Bay area.

Since you're looking for Ditto in Pokemon Diamond, you'll need to trade for one from someone who has a copy of Pokemon Pearl, Platinum, Brilliant Diamond, or Shining Pearl.