Smartphone case question

Has anyone here seen a smartphone case that can hold say cards/business cards without a lid / open book arrangement for using the phone? It seems to me that most smartphones are easily large enough whereby the case could have a few card slots at the back of the phone. The next problem I guess is that even if such a design actually exists, does it exist for other of the phones I currently have. The reason why I'm asking is that I've decided to carry business cards again (I'm self-employed), and I don't really want to end up with another object to carry around (for example, my coat normally holds my personal phone, my work phone, a small pen knife, keys and a pen, and even a slim business card wallet is still yet another item.