Shiny Xerneas is currently being distributed by Smyths Toy Store by serial code, until August 16th.
Then from 17 August until 15 September, return to get a code for Shiny Yveltal.
Simply ask for a code at the till, and then enter it in your game to unlock the Pokémon!
Update– Codes are being given away on specially printed cards and not receipts, which means limited stock! Get yours as soon as they can, before they’re gone!