Where do I get a ditto in pokemon diamond version

The only way I know is through the GTS. I don't think they come wild in Diamond or Pearl.

You can find a ditto in the wild.Try to go to the route near the Pokemon League.

i got mine from route 218, i also used a poke radar on the grass patches, (btw the grass is suppose to be sparkling). later on i put him in the day care with my nidoqueen, a few days later the old man outside gave me an egg, i think it's from those two pokemon.(i didn' expect that to happen at all!)

Backlots garden