Instructions
Add one Pokemon to your party with the Compound Eyes ability, which increases the chance of finding a held item on a wild Pokemon. Add another Pokemon with the ability Frisk, which allows you to see what a wild Pokemon is holding.
Go to the Safari Zone in "Pokemon Emerald" or the Trophy Gardens in "Pokemon Platinum" where wild Pikachus can be found. Use the Frisk ability to check if the Pikachu has a light ball. If he does not, run away from the fight. Repeat this process until you find a Pikachu that carries a light ball.
Use a "Pokeball" to capture the Pikachu that carries a light ball. The light ball will now be in your possession and you may equip it to one of your Pokemon.