If you want Mew in the Let’s Go: series, the only way you can obtain it by buying the PokeBall Plus device which is the newest addition to the franchise. By getting this ball, you can transfer the Pokemon from the mobile game to the Let’s Go game.
We are reminding you that once Mew is transferred from the PokeBall+, it can never ever be accessed again.
There is a proof provided by Nintendo Soup, where one user who did not like Mew’s transferred stats, deleted the save file because they thought that they will transfer Mew again and get better stats.
Here is the official statement from Nintendo:
“Mew can only be transferred ONE TIME, into one copy of Pokémon: Let’s Go, Pikachu! or Pokémon: Let’s Go, Eevee!.
Once it has been transferred to a game, it will no longer be available on the PokeBall Plus to transfer again to a different game.
Deleting a save file with Mew in it will also delete Mew. If you have deleted a game with Mew, you will not be able to get Mew a second time on a new game from the same PokeBall Plus.”
The only solution, should you do this, is to buy a second, new PokeBall Plus.