How can I get the light ball without the event, or if not how can I get the event?
In HG/SS, Red's Pikachu is always holding one, so you can steal it with Thief.
Here's Two ways
1.Reds Pikachu holds a Light ball so get a Pokemon to use Thief on it
2.Sometimes held by Wild pikachu wild ones can be found in Viridian Forest but it is quite rare for it to be holding one I say about 25% chance on it holding one