Ditto in platinum appears in the trophy garden. If you save before you talk to the owner of the trophy garden, you can talk to him. if he does not say he saw a ditto then restart the game and reapet it until you do. Hope this helps!!!
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The Trophy Garden is the only place to get Ditto in Pokemon Platinum, without trading or cheating of course.
No trainer has a ditto.. if you want Ditto for your National Pokedex, use the Poke Radar on Route 218
Use action replay code..
Once you have the national pokedex go to the Pokemon mansion and save in front of mr backlot then talk to him until he tells you there was a ditto in the trophy garden and use the pokerader (you HAVE to have the national pokedex)
Without an action replay, you can't mate dittos with legendary pokemon.