This is the one I use, and there are many other similar ones you can get for cheap on eBay, DealeXtreme, or Amazon, if you want a different color or style. Just fasten it to your thumb, set all aiming options to the highest sensitivity, and hold the 3DS like you would normally, and you're good to go!
I also recommend mapping the R button to using powers, and left/right on the dpad to switching them. It's kind of hard to reach into the lower left corner of the screen with the thumb stylus on your right thumb.
I don't use the thumb stylus anymore because I've gotten more used to the stylus controls, and find them just a tad more accurate. But for the first year and a half I played the game with this thumb stylus, and it worked great.