One of the more elusive Pokémon in Pokémon: Let’s Go, Pikachu! and Eevee! is Ditto. It tends to be pretty rare, but there are a couple of places in the games where you can find some.
The best place to look for Ditto is in the Pokémon Mansion on Cinnabar Island. Ditto only has a 1% spawn rate in most of the Mansion, but it has a 10% spawn rate in the basement area, where you find the Secret Key that unlocks Cinnabar’s Gym. 10% is still pretty low though, so it might be a good idea to use a Lure, Super Lure, or Max Lure to help bring out some of the area’s rarer Pokémon. With enough determination, you should be able to eventually find a Ditto.
The other area you can find Ditto in the games is in the post-game area, Cerulean Cave. Ditto can be found in any part of this cave, but only has a 5% spawn rate, so you’ll probably have more luck at the Pokémon Mansion. The Ditto that you’ll find in Cerulean Cave tend to be on much higher levels, however, so if you’re more concerned with finding a stronger Ditto, you might want to consider checking Cerulean Cave.