If you own either Pokémon X/Y or Omega Ruby/Alpha Sapphire and live in North America, you can now get your hands on a Shiny Xerneas, via the Nintendo Network.
To obtain your Shiny Xerneas, choose the Mystery Gift option from the main menu, then select the Internet option. Once you have done that, simply speak to a delivery girl in any Pokémon Center to receive your shiny legendary. The Pokémon will arrive at level 100, will have the Fairy Aura Ability, and will know the moves Geomancy, Moonblast, Aromatherapy, and Focus Blast. This distribution will last from May 11th to May 17th.
Pokémon X, Pokémon Y, Pokémon Omega Ruby, and Pokémon Alpha Sapphire are available for the Nintendo 3DS.