You can only catch Ditto in the wild, since Ditto can't breed more Ditto. So in short, you need to be really lucky.
Otherwise, in Gen 6 it's a good idea to have a few Dittos with different perfect IVs so you can breed them with whatever species you want perfect IVs in, and re-breed the offspring with a number of Dittos to get 5/6 perfect IVs.