Is there any legitimate way to acquire a Light Ball in Pokémon Black, White, Black 2 or White 2?
Wild Pikachu can be found carrying a Light Ball, but no Pikachu are encountered in Pokémon Black/White or Black/White 2. Pikachu can be transferred over from an earlier game (e.g. Pokémon Platinum), but items cannot. According to Bulbapedia, the Light Ball doesn't appear on any Dream World route or in the game Pokémon Dream Radar, and the Treasure Hunter on Route 13 doesn't give out Light Ball.
Is the Light Ball event-only in Gen V, or is there another way to find it without cheating?
Best Answer
Unfortunately, your research into the depths of Bubapedia has answered your own question, there are few ways to obtain a Light Ball. Serebii's ItemDex lists a 1% chance in B/W or 5% chance in B2/W2 for wild Pikachu to hold a Light Ball, but as you said, there is nowhere to (legitimately) encounter them.
Outside of events, there does indeed seem to be no way to obtain a Light Ball in gen 5. However, you could try browsing the GTS, although finding a Pichu/Pikachu/Raichu holding the desired item is probably unlikely enough without the fact that if you do manage to find one, it will probably be for an unreasonable or unobtainable request. Your best bet for obtaining one is probably to join a trading forum.