You need to finish the game and beat the elite four, then you go with rowan, and he will give you the pokeradar, but you need to have seen all sinoh Pokemon for you to receive the poke radar. When you have the poke radar go to route 218. Luck is important but keep trying it took me a hour to catch one.
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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.
The only way to catch a ditto is with a pokeradar. The only other way is through trading.
You must use the PokeRadar on Route 218. They are kinda rare though.
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.)
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.