Where do you find ghastly in Pokemon FireRed?

Answer 1

It is really important that you first obtain the Silph Scope item from the Team Rocket Boss in the Rocket Hideout (which is based inside the Celadon City Game Corner check the poster behind one of the Rocket Grunt's).

Once the item is obtained go inside Pokemon Tower at Lavendar Town and go upstairs until you reach a floor that is slightly foggy this is the place where you can find Gastly you can also find it on the floors above as well. Occasionally Gastly's evolved form Haunter will appear as well and rarely a Cubone.