Witch doctor sets?

The best chance to get any legendary items, whether set or not, is to have high magic find while playing on inferno.
The best monsters to kill are act 3 or 4 elites.
Ways to increase your magic find are: Nephalem Valor, Paragon levels, equipment with % MF (duh) and setting monster levels as high as you can while being able to take down elite groups.
There is no "guaranteed to drop this specific set piece" mobs or bosses like in World of Warcraft.
And to the best of my knowledge, those sets are not crafted.
If all else fails, use the auction house.

Good luck!