One of the strongest Pokémon in Let’s Go, Pikachu! and Let’s Go, Eevee! is the Dragon-type Dragonite. In previous games, players have been able to attain a Dragonite by first catching a Dratini before evolving it into Dragonair and then into Dragonite. However, Dratini isn’t found in Let’s Go in the areas those familiar with the Kanto region may try looking for it.
In the Let’s Go games, Dratini can only be found on Route 10. Once your Partner Pokémon has learned the Sea Skim Secret Technique, return to Cerulean City and from there travel east until you’ve reached Route 10. Here, there is some water that you previously weren’t able to travel on without the Sea Skim ability. Now that you have Sea Skim, surf on the water and travel up and down it until Dratini appears. Dratini only has a 4% spawn rate, so it could take some time for it to appear. You can use a Lure, Super Lure, or Max Lure, which increase the spawn rates of rare Pokémon, to try to get Dratini to appear. You can also try getting a large Catch Combo, which consists of catching the same Pokémon over and over (I’d recommend Magikarp, as it appears in this stretch of water a lot), which also increases the spawn rates of rare Pokémon.
There is also a chance that you’ll encounter a wild Dragonair on this stretch of water on Route 10, but its spawn rate is only 1%, so you’ll probably have more luck finding a Dratini. It is also technically possible to find wild Dragonite, but doing so involves beating the Elite Four and gaining access to the sky. Wild Dragonite can potentially be found in the sky of just about every area, but its spawn rate is so incredibly low, you’ll probably have more success trying to find Dratini or Dragonair. If you insist on trying to find a wild Dragonite, you can try to increase its spawn rate by using Lures or getting high Catch Combos as well.