Actually ehesister is right,
The Ultra Rare Soda is just another name for the Ultra Rare Juice as this is what it is referred to in the Official Pokémon Guide Book.
Anyway you can apparently create the Ultra Rare Soda in the Juice Shoppe. But currently it is unknown what two ingredients you need to mix together in order to create the Ultra Rare Soda. However the Guide Book does hint that one of the ingredients need is the Roseli Berry.
The other ingredient although remains a mystery as no one has experimented mixing all of the different types of berries to confirm it. Another reason why it is currently unknown is because it could be a berry that is not available as of yet in Pokémon X and Y.
For the Source from the Official Pokémon Guide Book confirming that you can create the Ultra Rare Soda and that one of the ingredients to create it is the Roseli Berry check here: