www.realhotstuff.com
Used them for years, they're actually located in America unlike all the Chinese sites. Just ordered one from them (I've had plenty of AceKards, I end up giving them to friends as gifts, LOL) and it came updated for 3DS.
In fact it now says "DSi (XL) and 3DS", which is fairly dumb. LOL. It still only plays DS games. The new ones also have some shiny sticker on the back saying they're genuine, as apparently people are faking flash carts now (Why would you even want to? The real ones are cheap enough...)