Then I suspect the Ditto is available at 7th Street, which is after beating Radomus.
I got Ditto after going back to the warehouse in Coral Ward immediately after destroying PULSE Muk in the factory, watch the cutscene where the Meteor Grunt says he's transferring the pokemon to a secret location, then rush back to the factory to see a Ditto being placed there which weren't there before. I then apply the usual method of obtaining it and BAM, job done.
For your case, I suppose "someone" has taken the pokemons to another place, thus my first sentence.