Theres no trick or anything that I know of, its just luck I guess. Just keep trying and moving grass isn't all that rare. Keep the sound up on your DS and you should be able to hear when the grass is shaking. The trick is getting to the grass without running into a Pokemon before you get to the patch of grass.
Though to be honest I think the electric spider Pokemon is the best electric Pokemon you can get in the game.
"Don't walk behind me; I may not lead. Don't walk in front of me; I may not follow. Just walk beside me and be my friend."