You can get it as a Mystery Gift on the Yellow Forrest route. Other than that, event Pikachu often hold one. I thinkt here's even a small chance that a wild Pikachu from Viridian Forrest will hold one. Enjoy your grind.
when tou battle red on his pikachu has a light ball you have to use theft to get it.
You give it to a pikachu to power up it's moves.
You can't get a GS ball in Soul Silver. You can only get it in a Japanese Pokemon Crystal. Sorry.
no
snorlax
silver apricorn
the silver apricorn
If you use the Pokewalker on the Yellow Forest route you may get it by taking 7000+ steps
latious
The Goldenrod Underground.
prof. oak
you can only get a cherish ball from Nintendo event