They are special events at stores like Toys'R'Us and GameStop. Special Pokemon like Arceus, Darkrai, and Celebi are distributed through them.
special events
I am afraid you can not.
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.
6.
by using wifi or wireless at a certain place
Celebi is only available through special Pokemon Events.
It's impossible to get without certain events/Hacks. Sadly, all of the events are over.
Action replay or Nintendo events
Usually Arceus can only be obtained through Pokemon events, and all the soulsilver events passed :(
Transfer it from your bottom game or get it from a Pokemon event. Pokemon events are under Wifi Conection but they don't happen often.
None. All of the Pokemon HeartGold and Pokemon SoulSilver events have ended, no more events for those 2 games. ~ 16 December 2012