As for Mewtwo it’s a normal legendary so it is able to be traded via GTS. Your best bet is to put up an extra legendary Pokémon if you have or one you don’t want anymore into the GTS asking for a Mewtwo (any fair trades I normally put up tend to go somewhat quickly). You can search for Mewtwo that other people have put up but it’s usually a waste of time as they often ask for something unreasonable or impossible. Though if you own Ultra Sun or Ultra Moon and have completed the game there’s a way you can even catch one yourself in the game. It can take a bit to find and you’ll want a high level team to battle it if you don’t have a Master Ball, but that’s an option too if you wanted to catch it yourself. (Message part 2 of 2)