I am afraid you can not.
special events
no. only events
A hack of Pokemon SoulSilver that makes the game harder and adds new Pokemon not in SoulSilver, along with new game events.
Well 1st of all your a dork because you missed they event
6.
by using wifi or wireless at a certain place
Celebi is only available through special Pokemon Events.
no they do not
It's impossible to get without certain events/Hacks. Sadly, all of the events are over.
They are special events at stores like Toys'R'Us and GameStop. Special Pokemon like Arceus, Darkrai, and Celebi are distributed through them.
Action replay or Nintendo events
Usually Arceus can only be obtained through Pokemon events, and all the soulsilver events passed :(