Photos with Mario was silently released on the 3DS eShop this week, allowing you to snap pictures of Mario (and his friends) in the real world, using special AR-enhanced eShop cards.  According to the application's manual, the special AR eShop cards will be available at Target, although there is no indication as to when they will be available.  In the meantime, you might try using the scans over at the AR Cards Database, made from the AR cards released in Japan.  The AR Cards Database has the three cards referenced in the Photos with Mario manual (Mario, Peach, and a Goomba), as well as an additional three cards (Luigi, Bowser, and a Koopa Troopa).

Note: To download Photos with Mario, just search for it by name on the 3DS eShop.

Link: AR Cards Database