You can only get number 251, Celebi, by hacking or going to the over by many years event in the states/wherever they do events.

Though, there's speculation that if there are GS Remakes, they'll release Celebi somehow. Which I hope they do, because it'll be one of the only ones I'll be missing in my Living Dex(Which, might I add, is up to 386 or so Pokemon owned =)).
I'm lucky-I went to the tenth anniversary convention and got Celebi. but it knows a HM move, so I need to know where the Fire Red/Leaf Green Move Deleter is before I can import it.
Fuchsia City

I have never been to an event for a special Pokemon-but i will get Arceus if it kills me.
Same, except, I'll just wait for the wi-fi event/event item if it happens.

And ifwhen they do the GSDS thing,i am going to try and get Celebi,event or no event.
Same, and hopefully it happens over Wi-Fi.
I think in G/S/C you have to get th GS Ball and take it to that forest...
Only Crystal, item didn't exist in G/S I don't think.
/EDIT: ... Hmm. So, the Platinum series of Special recently started it seems, and it seems to be interesting from the summaries... Also, even more interesting, it apparently mentions that there's a Battle Tower in Hoenn and Johto, but since it could just be referencing past events, since it's been to Johto before in the G/S/C arc... That's probably all it is.