It's called simply the Nintendo DS Web Browser. You can order it either through GameStop.com, or directly through nintendo.com. Should be $29.99.
GameStop and EBGames stores carry them, but ONLY the DS LITE version. This will NOT work with the original styled DS. If you have an original DS, you must order online through nintendo.com
PLEASE NOTE, though, which version of the Nintendo DS you have! There are two versions of the browser memory expansion (included), one for the DS Lite and one for the original (fatter) DS. The only difference is that the DS Lite version's GameBoy Advance-styled memory cart is too short to fit the original DS' design.
You CAN use the non-Lite version of the browser's memory cart with the Lite, though.
Just keep in mind that the DS is WiFi only, there is no wired connection. You'll need a hotspot. Most wireless routers work fine, and Nintendo also sells a relatively hassle-free USB dongle.