Instructions
Ask other players if they have Celebi. The simplest way to get Celebi in "Pokemon Ruby" is to swap Pokemon with another player. Gamers in America could receive Celebi for "Pokemon Ruby" during store events at one time. If another player has Celebi and is willing to trade, Celebi will join your collection after one trade.
Buy a "Pokemon Ruby" Japanese pre-order bonus CD. The bonus CD originally came with the Japanese release of the GameCube game, "Pokemon Colosseum," which you will also need for this method to work. Look at auction websites, second-hand game stores and online "Pokemon" boards to locate a copy.
Catch all 48 in-game "Pokemon Ruby" Pokemon.
Connect "Pokemon Ruby" to "Pokemon Colosseum" and insert the Japanese bonus CD. The game will register that you have collected all 48 Pokemon and upload Celebi to your game.