They are held by wild pikachu with a 5% chance so keep catching pikachus in the trophy garden or have a frisk Pokemon to see if it holds a light ball and then you will find a pikachu holding one eventually. Source: http://bulbapedia.bulbagarden.net/wiki/Light_ball#Light_Ball
The Pikachus in Mr. Backlot's Trophy Garden have a 5% chance of holding a Light Ball.
Source: Experience
On Route 202 I believe the route with the Trophy Garden Mansion. There is a 5% chance a wild Pikachu will hold a Light Ball. A Pokemon with the ability Frisk is helpful so you don't capture/ steal from the wrong Pikachu.
