Mewtwo is a Legendary Pokemon that is very powerful and often use due to it's strength in the game. It can be found anywhere where you can encounter wild Pokemon and has a 1/1000 chance of appearing, however you can make it easier to find Mewtwo by buying the "Kanto Legendaries" gamepass which will make mewtwo a 1/100 chance of encountering. You can also use the Legendary booster in the menu tab to make it a little bit easier as well.