How do you get haunter?

In red, blue and yellow. Search around in the Pokemon tower for a while even though you come across countless gastlys. In heartgold and soulsilver, look in either the sprout tower at night or the bell tower (i think also at night) for a gastly to evolve into a haunter. Haunters themselves can be found at night in the safari zone or route 8, according to bulbapedia but i have never found one myself.