Unlike Mew, you can catch Mewtwo in all three Game Boy Pokemon titles. It's located at the end of the Unkown Dungeon (which you can access from Cerulean City once you have beaten the game). Simply go north from Cerulean City to Route 24, then use Surf to swim south to get to the cave entrance. However, there is only one Mewtwo in each game -- so don't accidentally knock it out. While it's possible to catch Mewtwo using Ultra Balls (you will need plenty of them!), most gamers save their Master Ball for this occasion.