Nintendo has announced that the downloadable version of New Super Mario Bros. 2 will launch at might on August 19, 2012. Here's their full announcement:
"Be among the first to get your hands on the New Super Mario Bros. 2 game by purchasing and downloading it directly to your Nintendo 3DS system from the Nintendo eShop starting at midnight on August 19th.
"Don't forget Club Nintendo members can get double the Coins for registering the downloadable version of New Super Mario Bros. by 9/20!"
Alternatively, you can purchase the retail version on Amazon.com, or your local video game store.