Typically, only decks marketed to magicians have double backed cards. For mass market decks, such as Bicycle and Tally-Ho, you need to separately purchase the double backed cards from a magic shop (online or in-person).
That may have been true at one time, but I don't think it's true anymore today.
USPCC produced decks (e.g. Bicycle) typically have 56 cards as a standard. So besides a 52 pack and 2 Jokers, there are 2 extra cards. Many designers of custom decks now include gaff cards, like a double backer. So even many decks that are geared to collectors may include a double backer.
Playing card designers are getting smart, and trying to get some cross-over to make their decks appeal to magicians, even if they aren't the primary audience. I have a lot of custom decks produced by USPCC that are for collectors, and also have gaff cards like a double backer.
As kthrzne says, it just depends on what the creator decides to do with their extra 2 cards.