There was never a Celebi event outside of Japan, so you still stand a chance.
Mew event is over, but it means lots of people have them thanks to wi-fi (I have 2 :D)
You could look in the Trade Corner for Mew and Celebi, as people use the magic of cloning to make it that 100 legit pokémon suddenly become 1,000,000 legit pokémon, half of which are the same one :P
You can also wait for Celebi to be released in the English-speaking part of the world, eventually.
Before wi-fi, people just had to drive to the events....or cheat. Some people cheat.
Also, I don't know about before....but in Gen IV, you can get the National Pokédex Diploma if you've caught every pokémon available in-game - So Manaphy, Phione, Shaymin, Darkrai, Celebi, Mew, Arceus, Jirachi and Deoxys aren't counted towards the diploma...although you won't have 493/493 captured.