Where do u find dialga,palkia, and giratina in pokemon black version?

It is not possible to catch the legendary Pokemon Dialga (#483), Palkia (#484) or Giratina (#487) in Pokemon Black or White. The only way to get them is to use the PokeShifter to trade them from Diamond, Pearl, Platinum, HeartGold or SoulSilver.

Sunday, February 05 2012
Source: http://bulbapedia.bulbagarden.net/wiki/Giratina