evolve gastly.
you can't find haunter in Black
look in the Pokemon tower in lavender for haunter. note, haunter is hard to find in the Pokemon tower
It attracts a ledendary pokemon.
Haunter can be found on Route 8 or in the Safari Zone.
you get it over a trade.
curmen cave
this is also for heart gold
most likely in any of the higher up caves, or just train a ghastly to level 25, lazyass.
you have to use hm rock smash on rocks to find them.
You can find them by surfing and fishing.
You can find wild haunters in Pokemon tower and lost cave.
pokemon heart gold