to get dittos go to the trophy garden and talk to the man in the first room to the left, he will tell you a Pokemon licked him on the face that day, the Pokemon changes everyday. So, if you go every day, he will one day say ditto, (it took me abut 30 tries!) To do this you must have the national pokedex, which is achieved by finding every Pokemon in the game's original pokedex.
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Ditto only appears at the Trophy Garden on certain days. You can catch other rare Pokemon there on other days. Hope this helped! =) (Even on the right day Ditto is rare.)
there is no trainer with a ditto but you can get one with the pokeradar next to canalave.NOTE:it will take a very long time and are not easy to catch.
you can catch a Ditto if you talk to Backlot every day for a long time. He will say that he saw a ditto there. Then, you can catch a ditto in the trophy garden.
The Trophy Garden is the only place to get Ditto in Pokemon Platinum, without trading or cheating of course.
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