I seriously caught 5 boxes of pikachu's before I got 1 light ball. Also, here's a hint. If you have a pikachu and enter the Safari Zone, the encounter rate will increase dramatically. I was shcoked when I saw how easy it was to find a pikachu. I recommend going in with 1 pikachu, and then catch more till you have 6 pikachu's in your party. Then, Pikachu's will be as common as Oddish.