Easy. Just go to Serebii.net.
But, you could go to Cerulean Cave or the Pokemon Tower.
I've also heard rumors of a Gengar roaming the Victory Road.
Chat with our AI personalities
Usually in caves and ominous buildings
You find ghost Pokemon and rotom. And if you have any Pokemon game in your gameboy slot you can also find gengar.
you need to find the silph scope this will reveal the ghost to be a marowak you can then defeat it
Haunter.... There isn't that many ghost types in firered besides the Gastly line which you can find in Lavender Town's Pokemon Tower
Because Ghost Pokemon can't see in the dark.