I found the most Witchdoctors in the " Jolly Roger's lair " on Port Royal. If you're not sure where that is, Just go into the jungle ( The one by the river, NOT by the governer's mansion ) and continue ALL The way through and you should reach a Cave. Go into the cave and you'll be in the Jolly Roger's lair where there are some Witch Doctors and other kinds of skeletons making fires.
Hope that helps!
