Pier-Luc found an Etsy vendor with some quite clever, yet cheap, business (and credit) card sleeves. She also sells similar iPhone pouches and more.

PL went for the geek sleeve, so I couldn’t (…) but I happily indulged in the retro mix-tape one.

Card holder: $13.50.
Not having to jam a wallet in your back pocket when you go out on the town: priceless.