http://www.newsweek.com/pokemon-latios-latias-event-distribution-best-buy-serial-code-1100072

There are only a few months left in The Pokémon Company’s year of Legendary distribution events, and in September trainers can get their hands on either Latios or Latias.

From Sept. 2 through Sept. 29, Pokémon trainers in North America can receive either of the Psychic and Dragon-types through a serial code from Best Buy. Like in past distribution events, your base game will determine which of Pokémon you receive, along with their level and move set.