I know Kawaii Terlor has already given an answer, but I feel there are a couple of things he missed out.
Firstly, if you have a Electric Friend Safari with Pikachu in it, then use that instead of Santalune Forest or Route 3, as you have a better chance of encountering Pikachu. If not, then it might just take a little longer, as you will be encountering Pikachu less.
Now lead with a fainted Compound Eyes Pokemon (e.g. Galvantula or Butterfree). It is better if it is fainted, since its ability will still raise the chance of the wild Pokemon holding items, but you won't be sending it out first in battle.
The second Pokemon in your party should have the ability Frisk and the move Thief. Noivern or Pumpkaboo are options here. Since Butterfree (or whoever your lead is) is fainted, then Noivern will be the first Pokemon you send out. Frisk will tell you immediately whether or not Pikachu has a Light Ball. If it does, then you can use Thief to take it for yourself.
I have used this to get many items off wild Pokemon - Lucky Eggs, a Light Ball, Black Sludge, Light Clay, and it works wonders.