EA and Dr. Pepper have been running the Exclusive Content for a while now, but according to the Dr. Pepper Promo Site, these items will be expiring January 31st. If you want to redeem any codes, or search for any remaining specially marked Dr. Pepper bottles, you should do it soon! There is no telling if this promotion will be extended again.
Another option is to check your “Buy Mode” within the game to check if you can snatch some of the Dr. Pepper items for free from shop mode. There are only a limited amount of items however.