You must use the PokeRadar on Route 218. They are kinda rare though.
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You can catch a Ditto in Pokemon Diamond after you obtain the National Dex and use the Poke Radar in the grass in route 228(or 227). Dittos have appear about 20% of the time.
you can catch dittos in Pokemon diamond and pearl with poke radar in route 218 but its pretty hard to catch them + find them. :)
The only way to catch a ditto is with a pokeradar. The only other way is through trading.
I'm afraid you cant get a Ditto in Pokemon Sapphire. You can only get ditto in Emerald. However you can also get ditto in Diamond and Pearl.
you cant catch ditto in the safari zone...you catch him in the Pokemon mansion in cinnabar