Yes, you can find a Ditto in Pokemon Pearl/Diamond/Platinum. In order to be able to find Ditto in Pokemon Pearl/Diamond/Platinum, you must have already beaten the Elite 4. After you have beaten the Elite 4, you need to have gotten the National Pokedex. To get the National Pokedex, you have to have SEEN all 150 Pokemon in the Sinnoh Pokedex. After you have done that, if you go and talk to Professor Rowan, Professor Oak will interrupt him and give you the National Pokedex. After you get the National Pokedex from Professor Oak, you need to fly to Pal Park. Once you get there, Professor Oak should be right outside. Talk to him and Professor Oak will give you the Poke Radar, which is the necessary item in order to be able to get Ditto in Pokemon Pearl.
After you get the Poke Radar, you need to fly to Canalave City and head east through the gatehouse onto Route 218. Once you get onto Route 218, you need to walk into the tall grass. After you walk into the tall grass, set the Poke Radar to the Y Button and start pressing the Y Button when you are in the tall grass. You should see several clumps of the tall grass shake and some of them might have white sparkles on them. If you walk onto a clump of grass that had a white sparkle on it, you might run into a Ditto.
DIttos are somewhat rare on Route 218, so don't be discouraged if you don't get one right away. The first time I looked for one, it took me about an hour and a half to get it.
If that doesn't suit you, you could also migrate it over from a Pokemon Fire Red or Pokemon Leaf Green game.
And if migrating doesn't suit you, you can always ask one of your friends who plays Pokemon Diamond or Pokemon Pearl or Pokemon Platinum to trade you his/her Ditto so you can have one.
And if you don't have any friends, or you live too far away from your friends, you can always try to trade for one off of the GTS.
Hope this helps :)