Action replay or Nintendo event. I used action replay because Nintendo doesn't give them out often.
Its much easier to trade for it, You need a special event to get it in your game
If you are talking about the one in Canalave City, which is the only one I can think of, you have to go to a special event and get a key. Inside is where you can find darkrai.
you get celebi on a gba game such as... ruby Sapphire emerald fire red leaf greenYou cannot catch Celebi in Pearl. Celebi is a Special Event accessible Pokemon only.
event or action replay
You can search for special event catering on the website Special Events Catering. They have great listings.
new moon island (first you have to do the darkrai event first but to do that you have to do the creisila event and the shaymen event)
unfortunately if it is the same as the azure flute you wont be able to unless you have action replay or event :(
Arceus is a special event pokemon. The only way to get it is to get it at a special event or to hack the game.
Fight everybody in the riolu event and a cherim will be there
You dont, you have to either get it from an event or action replay
i got arceas with event
you can only get it in a Nintendo event