You can get more by winning the lottery. However, Xerneas is the easiest to catch, because it wants to join you. Mewtwo, Zygarde, and the legendary birds are also easy to catch. Just buy Dusk Balls; most people catch them around 10 or so balls. Its really, really easy to catch legionaries since Gen V. So, I suggest saving your Master Ball(s) for shiny Pokémon :).