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)
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At the trophy garden, rarely. it's in Pokemon platinum.
The Trophy Garden is the only place to get Ditto in Pokemon Platinum, without trading or cheating of course.
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 find it at random days in Pokemon Mansion in route ---, talk to Butler (or whatsoever) and he will say random Pokemon is in the garden, if is ditto, go get it!
You can not get a Turtwig egg in Pokemon Platinum. But if you breed a Turtwig with a Ditto, then it is possible.