You won't be able to find it In-Game, like Lugia or Ho-Oh.
You'll just have to wait for a WiFi Event to come out in America, featuring Celebi.
Whenever that happens (make sire its in the first slot of your party), go "talk" to the Ilex Forest Shrine, and your "Childhood Friend" will appear.
More info about this event is on serebii.net ;)
Celebi
If you go to the Ilex Forest on a certain day in the summer, there is a celebration I think for Celebi which is when celebi will appear. You can catch it then.
You can only find celebi through an event in the ilex forest.
Celebi is the protector of the Ilex Forest.
You need to get the event Celebi and then go to the shrine in Ilex Forest.
The gaurdian of the Ilex Forest is Celebi.
Go to the Ilex Forest Shrine of Celebi and tamper with it. You must have Celebi walking with you. It will take you back in time and you will have to face the leader of team Rocket.
You can not get Celebi in the Ilex Forest. The only way to get Celebi was from a Wi-Fi Connection with GameStop or EBGames. But the event is now over.
You dont, but in japan they have a celebi event going on right now. Check out serebii for people who may have it and trade them.You can find celebi in the ilex forest during the celebi event.
Go to a shrine in Ilex forest and after that you time travel
You can not get Celebi in the Ilex Forest. The only way to get Celebi was from a Wi-Fi Connection with GameStop or EBGames. But the event is now over.
You get it in Ilex Forest.