In Ruby, Sapphire, Emerald, Colosseum, XD, Diamond, Pearl and Emerald, you would have to trade mewtwo from another game version. For the rest, you would have to catch Mewtwo by yourself. Mewtwo came out since the first generation.

The best type of ball you should use to catch Mewtwo is either ultra ball, or if you did not use your master ball on Latios or Latias or any other legendary pokemon yet, and you caught all, then use it!

For Red, Blue, Yellow, leaf green, fire red, soul silver and Heart gold versions, you can find Mewtwo in the cerulean cave.

For Gold, silver and Crystal versions, you can catch Mewtwo with the time capsule.