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.
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To get a Gengar, you must trade a Haunter and it will then evolve into a Gengar.
you can't find haunter in Black
There's a 30% chance you can catch a level 19 Haunter in the grass on Route 8 at night. You can also catch level 16 or 17 Haunter in the forest region of the Johto Safari Zone at night.
just catch a gastly in the tower before the first gym and level him til he evolves
you cant u need to catch a ghastly in the sprout tower and evolve it. it is in there at night only.