From July 1st to the 24th, Shaymin is available through the Nintendo Network (so everyone can get it -- no need to live close to a game store or anything)! Available for Pokémon XY and ORAS.
From the official Pokémon site:
After that, you just need to go to any Pokémon Center and speak to the delivery girl to receive Shaymin.
To get the Gracidea Flower (the item that turns Shaymin into Sky form) on XY you need to show Shaymin to a girl in Snowbelle City; for ORAS, go to Route 123 and show Shaymin to a man in Berry Master's house.