In B/W you can find a Zen Mode Darmanitan in the Desert Resort. They are right outside the enterance to Relic Castle and need to be given a Rage Candy Bar in order to be awakened. Going to the Dream World would be pointless because I've looked and it's not on the lost of Pokemon at appear there. You can't get a Zen Mode Darmantian in B/W 2 without trading from B/W.
Source: Looked on Bulbapedia