They are found in shaking spots in the grass at Floccesy Ranch and Route 20.
Source: Route 20 and Floccesy Ranch and also experience. I asked my brother what Pokémon he wanted, and I was playing White 2, and he asked for a Dunsparce. OUT OF ALL THE POKÉMON-
The easiest way to do this is to get a specific Merchant at Join Avenue, the one that sells Escape Ropes, Potions etc and get a LOT of the best kind of Repel there is. Then use it and keep on using more as needed, looking for shaking spots.
Source: Experience with catching the Dunsparce my brother wanted.
Hope I helped!