you get a light ball from a wild pikachu in the Safari Zone. you have to breed two pikachus (or pikachu and a raichu, etc.) and one of the pikachus has to be holding the light ball. Pikachu is rare, and a pikachu witha light ball is even rarer(is that even a real word?) so it may take a while. my first pikachu in emerald had a light ball. i just kept it in the pc untill i found out what it did.