Ditto can spawn in the following locations:
Asado Desert, North Province (Area Three), East Province (Area One, Two and Three), South Province (Area One, Two, Three, Four, Five and Six), West Province (Area One, Two and Three), Glaseado Mountain and the Pokemon League.
The highest encounter rates are at East Porvince (Area One, Two and Three), Southern Province (Area One, Two, Three and Six), West Province (Area Two and Three), Glaseado Town and The Pokemon League.
Zoroark can spawn at the following locations:
Tagtree Thicket, Casseroya Lake, North Province (Area One and Two), Eastern Province (Area One and Two), Southern Province (Area One, Two and Four) and West Province (Area Three)
The encounter rates are the same for all the locations Zoroark is found at.
Since Zoroark will always be disguised as a different Pokemon, you will need to send a Pokemon to battle Pokemon in the overworld and use a move against it without fainting it. If it transforms into Zoroark, you can obviously catch, and if it doesn't you'll need to run away from it and try another Pokemon until you find a Zoroark.
This also applies to Zorua if you prefer to catch it instead and evolve it starting at level 30. Here are all the place Zorua can be found.
Source 1
Source 2
Hope this helps!
