If you live in the US, I suggest realhotstuff. Two day free/USPS shipping to Illinois.
As for the cart, I suggest the DSTwo. In addition to having DSTwo software for GBA (good to very good) and SNES (average to good) you can also run 386/DOS (good to very good, depending on whether the game/app is real or protected mode) software on it. It can also have the firmware updated even if you have already updated your 3DS and it won't recognize it as a valid cart (you just rename the firmware update a certain way, insert the DSTwo, turn on the system, and wait two to three minutes). As for DS game support, it runs just about everything perfectly (even Metafortress games). It also has DS savestate support (multiple), multiple save (.sav) file support, an ingame text reader that supports multiple text files per game, and the usual cheat engine. The only two catches are that the slow motion feature doesn't work with some games (like Tetris) and the cheat names won't scroll if they are too long to fit on the screen (so you will only ever see the first part). Finally, they actually released the full source code to their SDK (including the DS/ARM side) which will make it possible to write even better homebrew that takes advantage of the DSTwo's onboard CPU.
As for the cart, I suggest the DSTwo. In addition to having DSTwo software for GBA (good to very good) and SNES (average to good) you can also run 386/DOS (good to very good, depending on whether the game/app is real or protected mode) software on it. It can also have the firmware updated even if you have already updated your 3DS and it won't recognize it as a valid cart (you just rename the firmware update a certain way, insert the DSTwo, turn on the system, and wait two to three minutes). As for DS game support, it runs just about everything perfectly (even Metafortress games). It also has DS savestate support (multiple), multiple save (.sav) file support, an ingame text reader that supports multiple text files per game, and the usual cheat engine. The only two catches are that the slow motion feature doesn't work with some games (like Tetris) and the cheat names won't scroll if they are too long to fit on the screen (so you will only ever see the first part). Finally, they actually released the full source code to their SDK (including the DS/ARM side) which will make it possible to write even better homebrew that takes advantage of the DSTwo's onboard CPU.

