Instructions
Find someone who attended a promotional event held by Nintendo that gave away Celebi. She can have her Celebi in one of the following Pokemon games: "LeafGreen," "FireRed," "Sapphire," "Ruby" or "Emerald."
Offer him Nintendo Event Pokemon, Legendary Pokemon, Shiny Pokemon or a Pokemon infected with Pokerus. Depending on how much he values his Celebi, you may have to offer multiple valuable Pokemon for the Event Celebi.
Arrange a date, time and place for you to trade. If the person you're trading with owns "Sapphire," "Ruby" or "Emerald," complete the mission in Sevii Islands to obtain the Sapphire and the Ruby gems. You and the person you are trading with must have the National Pokedex.
Use a Game Boy Advance link cable to connect your game with her game. Start both games and go into any Pokemon Center. Go to the top floor of the Pokemon Center in both games and talk to the woman to the very right. She will lead you to the trading room.
Offer the Pokemon you agreed to trade for the Celebi. Accept the trade once he offers the Celebi. The trade will only take two or three minutes to complete.