Sorry, it is impossible to catch Celebi in pokemon crystal without hacks. The same goes for Mew, Mewtwo, Zapdos, Moltres, and Articuno. I've played this game through and through.
Unfortunately, there was an event, which passed a long time ago.
>It serves as the guardian of Ilex Forest and is only able to be caught from the wild in Pokémon Crystal with an event activated by the GS Ball. -Bulbepedia
You can also hack.
Hope This Helped :D