Instructions
Connect to Pokemon's online trading function. This can be found in the game in Goldenrod down at the Global Trading Station building. Activate an in-game computer and find someone who will trade a Celebi. Online trading is only available on Pokemon games made for the Nintendo DS.
Attend a Gamestop function where Celebi is being given out. The events will be listed on the Gamestop website. While attending, simply connect your copy of the game to the Gamestop computer and allow the trade. Celebi will be given for free.
Use a Gameshark and input the code "91C089DA." Begin the game and talk to Kurt in New Bark Town. A Celebi will be added to your captured Pokemon automatically. This will only work in Pokemon Crystal.