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.
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To get a Gengar, you must trade a Haunter and it will then evolve into a Gengar.
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.
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.
Sorry to disappoint you, but Haunter doesn't evolve by level. It can be evolved at any time after it has become a Haunter, but only if you trade it to someone (another one with a DS not in game). It will then evolve into Gengar and then you trade it back.