you have to either import it from Pokemon emerald, use an action replay code or trade it.
Without Action Replay or PKMN Sapphire you can trade with PKMN Emerald.
You need to trade or action replay
Trade from Pokemon leafgreen or firered
trade from another Pokemon emerald game.it is the only chance to obain another starter without action reply
trade it or get a action replay
you cant but you can trade it from ruby, sapphire, and emerald or use the action replay.
not without a game shark or action replay but you could trade from emerald or sapphire.
You have to trade it from a different game or you need action replay for the cheat code. You can trade if from Pokemon Emerald, Sapphire, and Ruby to Pokemon migration center.
You Have These Options -Trade From HeartGold Or SoulSilver (Real Pokemon Game) -Migrate From Emerald -Action Replay Pokemon Modifier -Gameshark
The only way to get celebi is to use an action replay, or trade it from Pokemon colosseum/gale of darkness.
You would need to trade with a friend who has Mew or anyone else.