Sheer luck. You'll have to catch one holding it, and only about 5% of pikachu actually have it. It may take you a while, or it may take you no time at all. It all depends on how lucky you are.
You can find a lightball on wild Pikachu, but it is very rare.
there is actually a cheat for getting a lightball in Pokemon pearl and diamond it is for an action replay
Lightball
CATCH PIKACHU UNTIL YOU CATCH A PIKACHU THAT HOLD THE LIGHTBALL.
you need a light ball and a male and a female Pikachu. Make the male hold the lightball then stick them both in the daycare, when you have an egg the Pichu will know volt tackle
By catching a female pikachu that holds the item lightball then breed
Heres a good way, find a male cherrim and a female pikachu, have the pikachu hold a lightball (you can find pikachu randomly holding it in the wild) and leave them in the daycare, an egg will eventually appear in the daycare, and the egg will have a pichu that knows volt tackle!
catch alot of pikachus or use thief on them until one 5%pikachu has the light ball mostly i caught one in Pokemon diamond and got the light ball
i don't think you can you would need to trade from ruby sapphire or emerald
either get a volt tackle pikachu from the walker in yellow forest or breed a pikachu holding a lightball, the hatched piccu will have volttackle
The lightball is an item that 5% of the pikachu you battle will be holding. To get it, you either catch every pikachu you see, or use a move that steals items from pikachu. However, like 50% of the pikachu are holding oran berries.
You can find them in the Yellow Forest (Pokéwalker Route). You might find some wild Pikachu holding them too.