Fighting-type pokemon have come a long way since their somewhat abysmal roots in the first generation of Pokemon, and it pays to make your fighting-types as strong as possible. Why not up their moves with a Black Belt?
Finding a Black Belt in Pokemon Black and White 2 is relatively easy, so long as you only want one, though you'll have to wait until you've beaten the game once to get it. After you have access to the many post-game areas, travel to Icirrus City and visit the Pokemon Center. A woman in here will give you a Black Belt.
Getting another Black Belt is somewhat trickier, as you need to get it from either a wild Sawk or a wild Throh. Visit Pinwheel Forest and hunt through the grass, using Thief on either of these pokemon that you encounter. One of them will, eventually, be holding a Black Belt.