Some Pokémon must have a little more utility than others when it comes to Pokémon Brilliant Diamond and Brilliant Pearl. One such Pokémon is Ditto, an extremely useful Pokémon for people out there who like to raise their own Pokémon.
Ditto is able to take the form of any Pokémon and mate with any Pokémon, which means you only need to find one Pokémon, as long as you have a Ditto. Ditto's importance to breeding mechanics almost means that it is rare by design and can be difficult to obtain.
In Pokémon Shining Diamond and Shining Pearl, Ditto can be found in two locations. On Route 218, there is a 20% chance of encountering a Ditto while using your Poké Radar. Due to the area, the Ditto found here will be around level 29 or so.
After that, the Route 212 Trophy Garden behind the Pokémon Mansion can also produce a Ditto. Not only is the Ditto here a lower level of only 16 or so, but the chances of finding it are also much lower. You have about five percent to find a Ditto in this area, so it's best to search on Route 218 with the Poké Radar.