PokéBeach.com Forums

Sorry to break it to yopu, but there is no such thing as a legitimate shiny celebi (there are pieces of code in place for all legitimate events that prevent it from ever being shiny). Any such pokemon would be hacked/modified. Legitimacy/legality probably isn't that important to you, but seeking out things like this can get you ripped off rather easily, if not looked down upon should you try to use it in battle.

Also, if you ever enter a tournament with it it could get you disqualified (just a fair warning).