go to pokemon tower or lost cave in leaf green(lost cave is on five island(pokemon tower is in lavender town))
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you can't find haunter in Black
You can't catch a Haunter in Emerald. You can trade for one, you can find a Duskull/Dusclops at MT Pyre, but you can't straight out find a wild Haunter.
You can find Haunter in SoulSilver in Route 8 at night. Additionally you can catch a Gastly at night in the Sprout Tower or the Bell Tower and then you can train it until Level 25 which is when it'll evolve into a Haunter.
You have to find your friend, you trade your Haunter with another Pokemon and when the trade is done Haunter will evolve.
You can't find gengar in the wild, you have to get a haunter and trade it.