Well if you got the Celebi from the event then you can use the Poke-Transfer so you can send Celebi into Pokemon White/Black.
Hope this helps =P
There was an event, but it has long since passed...
Bulbapedia/ Event Celebi
There was a special event to get celebi but it is finished.Mybe there is another one.
How to change your username
How to add a user avatar
Why was my post hidden?
Flags vs Downvotes
Official PokemonDb League
Type a Pokemon name, type, move or ability to see instant results.