You can't, giga impact is a Gen 4 move.
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Giga impact
From my experience, you cannot find or buy the move Giga Drain In Pokemon Emerald. It is only a learned move. If you have a Shroomish, it will learn Giga Drain in one of its advanced stages.
Imran: Yes Snorlax can learn giga impact. When I caught the Snorlax Lvl50 that was sleeping in front of Digletts Cave it knew Giga Impact in Pokemon Heartgold and Soul Silver. I do not know about other pokemon games in which you can catch Snorlax that they would know Giga Impact or not. But of course you can teach Snorlax Giga Impact with a TM. So, yes Snorlax can learn Giga Impact.
theres no such thing as a giga rod, sorry. if you mean the super rod you get it from a guy in a house in mossdeep.
Tauros, Aerodactyl, Snorlax, Donphan, Electivire, and Regigigas learn Giga Impact by leveling up.A lot more learn it by TM.