Here are some ways to get Mewtwo in those games:
* In Pokémon Red, Blue, and Yellow: After defeating the Elite Four, you can go to the Cerulean Cave, located north of Cerulean City. There, you'll find Mewtwo.
* In Pokémon Let's Go Pikachu and Let's Go Eevee: Mewtwo is found on the Island of the Giant, which can only be accessed after completing the main story.
If you're playing FireRed, there are no ways to obtain Mewtwo legitimately. However, there are some ways to obtain it through cheats or glitches, but these are not recommended.