The elusive Mew has been known as a hard-to-get Pokemon since the original Pokemon Red and Blue games, but the newest games, Pokemon Let's Go, Pikachu! and Pokemon Let's Go, Eevee! also have the ability to get this mythical Pokemon. There are two ways, currently known.
Pokeball Plus
Announced during the Nintendo Treehouse at E3 2018, all Pokeball Plus accessories come with a Mew already embedded into the device right out of the box. This can be obtained either as a bundle with the game or individually.
No details on the Mew players get as of right now, but this will be updated when more details emerge.
Pokemon GO
There is currently no way to bring Mew into the game from Pokemon Go. There may be a way at a later time, but event-based Pokemon are currently locked to stay within Pokemon go right now. This includes any of the Pikachus with hats, as well as those related evolutions and Squirtle's evolution line with sunglasses. The only exception is Meltan and Melmetal, which is obtained using the new Mystery Boxes.
Currently, you can bring Mewtwos and the Legendary Bird Trio into the game, as they're caught in raids instead of special research.
Keep a look out for Mew to be a likely event within Mystery Gift thati could be obtained via code or through wi-fi, similar to past Pokemon game events.
This article will be updated as more details are added, but in the mean time, check out how to download Pokemon Quest on the Nintendo Switch that was announced at the same times as Let's Go and then get cooking right!