Haunters can be found in the old cheatu at the end of Enterna forest and you can get it in a trade in with a girl in snowpoint city for a medicham.
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you can't find haunter in Black
Haunter evolves into Gengar by trading. Since the GTS is not functional in Pokemon Diamond and Pearl anymore, you will need to ask a friend, who has a copy, to do a trade-back with your Haunter.
you can trade it at the guy at snowpoint city
Gastly evolves to Haunter at level 25. Then, Haunter evolves into Gengar by being traded.
you trade him. and he will evolve into gengar. you trade him. and he will evolve into gengar.