Nowhere. You can't find celebi in crystal at all unless you have a Gameshark and cheat
codes. There's no easy, legal way to get one. There's a whole freaking complicated glitch
you can go through to get a celebi, but if you don't do it just right your game will
crash. Plus if you don't have Pokemon Stadium 2, you won't be able to delete any glitched
pokemon you get during the glitch. Sucks, doesn't it? I've been wanting to get a celebi
for a while, and when I found out I couldn't without cheating, I was really annoyed.

you can get a celebi! by pokemon events , they will give u a GS ball but i dont think they
hold crystal events anymore :P