To find tm 19 (Giga Drain) You must first have a grass type pokemon FIRST in your party. Then Surf South of Mount Pyre. You will see a girl looking at the trees near some soil. Talk to her, and only if you have a grass type pokemon in your party FIRST, she will give you Tm 19.
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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.
You can't, giga impact is a Gen 4 move.
Head south of Mt. Pyre, and you'll see a girl next to the soil. If you have a Grass-type in your party, she'll give you TM 19 - Giga Drain.
Surf to Route 123 from Mt. Pyre and talk to the lady near the soil facing a tree.
TM19 Giga Drain is given by the Celadon City Gym Leader Erika after winning a battle against her.