Those that purchase a copy of Pokémon X or Pokémon Y at Tesco stores in the UK will receive a unique serial code to get 12 Quick Balls. Players can enter their code in their game to receive this set of Quick Balls. Quick Balls are particularly effective when trying to catch a wild Pokémon at the beginning of battle and they’ll help players start building their Pokémon collection right away! Pokémon X and Pokémon Y game packaging at Tesco stores have stickers on them to indicate they have the serial code inside.

Source: http://www.pokemon.com/uk/pokemon-news/vg_pokemonxy_quickballs-2013-10-03/