Either catch bagons (smallest room in meteor falls) or horsea(left of pacifidlog) there is a 5% chance of catching one with a dragon scale. Attach the dragon scale to a seadra and trade it to get a kingdra.
Also if you have a pokemon that knows covet I recommend using it. That way you don't have to waste pokeballs catching Horseas (or Bagons) you don't need.
