i don't think you can you would need to trade from ruby sapphire or emerald
You can find a lightball on wild Pikachu, but it is very rare.
Lightball
CATCH PIKACHU UNTIL YOU CATCH A PIKACHU THAT HOLD THE LIGHTBALL.
By catching a female pikachu that holds the item lightball then breed
A light ball you can't technically find anywhere. You could get one if you caught a Pikachu and it was holding one. Pikachus have a 10% chance of being found holding one.
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
there is actually a cheat for getting a lightball in Pokemon pearl and diamond it is for an action replay
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.
Pikachu evolves with a thunder stone.
A wild pikachu Has a 5% chance of holding a lightball when you battle it. My best recommendation is to get a stantler with the frisk ability and wait for the light ball pikachu to appear then capture it.
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.