you can find ditto on route 13, 14, and 16, cinnabar mansion, or the unknown
dungeon(cerulean cave)

to get a ditto go to fuchia city and go to the left until youto a path where you see a
girl camper.once you fight or dodge all the trainers go to where you see like 2 or 3
bikers and one riding down and up.fight them and go up until you see a cut place.use cut
go into the gr*** and like the first pokemon you will see is like a level 23 ditto