Well, in SS, Celebi was an event Pokemon. You had to get it from Mystery Gift. The event long past. In Crystal, the Japanese versions allowed you to get a GS Ball within a mobile event, and if you take it to the Ilex Forest, Celebi appears, however, it is not the same as in the remakes. You could only get it through event, and since it long past, you can now only get it by hacking. And in SS, Ilex Forest has some relations to it, if you take the event Celebi (event only), to the shrine, Celebi will make you time travel and see the flashback of Giovanni.
Hope this helps! And if I was unclear, you now cannot get Celebi unless by hacking.