Since the only way to obtain a Celebi through standard game play is from an official Nintendo event, a Celebi can be obtained using cheat codes or the Celebi Egg glitch. To perform this glitch, the you need an egg that knows Beat Up as its third move upon hatching, a bad clone and any five other Pokemon.
Obtain the egg
The egg that knows Beat Up as its third move upon hatching can be obtained by mating a male and female Sneasel at level 57. Both Pokemon must know the same four moves in the same order, with Beat Up in the third position.
Obtain the bad clone
To obtain a bad clone, you must exploit a glitch that exists in the Pokémon game series for cloning Pokemon. The bad clone has a name made of only question marks or is completely blank, is level 0 and has a different gender than the prototype.
Obtain any five Pokemon
These five Pokémon should be unimportant Pokémon, possibly Pokemon the player has obtained while attempting to get a bad clone.
Put the egg into a box
Deposit the egg into a box. The five unimportant Pokemon and the bad clone should be in the party now.
Put the bad clone in the Day Care
After giving the bad clone to the Day Care Man, take it back and the clone is a ?????.
Put the ????? at the top of the party
Once this Pokemon is at the top of the party, go to a PC.
Move the Pokémon with the "Move Pokemon without mail" option to the top of the party
Using the "Move Pokemon without mail" option to move any Pokémon from a PC Box to the top of the party should change its name to the name of its original Trainer. You now have seven Pokemon in your party.
Return to the Day Care
Ask the Day Care Man to raise the ?????. Then, return to the PC.
Deposit the first and second party Pokemon
Deposit the first and second Pokemon from the party into the Box.
Use "Move Pokemon without mail" again
Use the "Move Pokemon without mail" to move the egg from the PC Box to the top of the party.
Deposit the next four party Pokemon
Desposit the next four Pokmon in the party into the Box, but not the egg.
Withdraw any Pokemon not used for the glitch
Withdraw from the PC Box any Pokemon not used in the glitch.
Walk or bike around until the egg hatches into a Celebi. This Celebi is level 0 and temporarily unplayable.
Allow the Day Care to raise Celebi to level 2
Allow the Day Care Man to raise the level 0 Celebi to level 2, and then it works the same as any normal Celebi.