Is there a way to obtain event pokemon after the event?

I'm working on completing my pokedex in Y, and I'm looking at the list of legendaries and realizing that while I've got about half already, there are a few left to get that I'm told I can't get except through an event (Celebi, Deoxys, Manaphy, Darkrai, Arceus, Keldeo, Meloetta, Genesect).

So... is there any way for me to get them? Or do I just have to wait until Nintendo decides to throw out another event?