Answer 1

The only legitimate means of getting Celebi is through a wifi event, or the japanese bonus disc of pokemon colosseum. However, the event is over, and the disc is very rare (and likely expensive). You could find somebody willing to trade, or challenge someone to a battle and get them to wager Celebi.

Comments

DT, don't forget that using PokéDex isn't allowed in Wi-Fi battles;))

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I think he means challenging somebody who owns Celebi and challenging them to a battle, and they have to give up a Celebi if they lose.

Answer 2

You get celebi by an event which has passed so you can't get him sorry.