^ Actually, Gengar is a dongle pokemon with any GBA game (found in Old Chateau in the room with the painting of the "eyes"). But if you don't have a GBA game, there is a trainer (a psychic I believe) with a Gengar somewhere on the first floor of Victory Road. Just an FYI, every single non legendary pokemon in the Sinnoh dex can be found on at least one trainer somewhere in the game.