Lewis puts a mark in his book and looks up. He says "The best place in Kanto to find ghosts is in Lavendar..."
He looks towards the haunted tower. "I haven't ever actually /tried/ to catch any ghosts myself. I've heard rumors about that tower over there." He points to it. "Ghost pokemon are attracted to tombstones. They say that sometimes ghosts will emerge at night. The best conditions to attract ghosts are dark, foggy, starless, moonless nights."
He sighs and says "Ghosts are no joke, though. I wouldn't try to use a 'puff to try and catch them."