Some Pokémon must have a little more utility than others when it comes to Pokémon Brilliant Diamond and Shining Pearl. One of those Pokémon is Ditto, a massive useful Pokémon for folks out there who like to breed their Pokémon.
Ditto is able to take on the form of any Pokémon and breed with any Pokémon, which means that you only ever need to find a single Pokémon, as long as you have a Ditto. The importance of Ditto to the breeding mechanic almost means it is rare by design and can be hard to get.
In Pokémon Brilliant Diamond and Shining Pearl, Ditto can be found in two locations. On Route 218, there is a 20% chance to encounter a Ditto while you are 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 that can be found behind the Pokémon Mansion can also yield a Ditto. Not only is the Ditto here a lower level of just 16 or so, but the chances of finding it are also much lower. You have a roughly five percent of finding a Ditto in this area, so are better off searching on Route 218 with the Poké Radar.