I bought an M3 DS Simply flashcard from Divineo.cn (good experience, received in 3 days via EMS shipping) and have started making skins for it. Loads of fun. A word of advice to anyone looking to buy one, don’t use Sandisk MicroSD (TransFlash) cards. You’re likely to run into compatibility problems on certain ROMs/apps, and the transfer speed is supposed to be sub-par. Word on the street is that Kingston cards are fine, but only the ones manufactured in Japan. So check the back of the package before buying. The other countries producing Kingston are Taiwan and China.
The M3 and R4 cards are essentially the same products, but release their firmware updates separately. The R4 is cheaper, but its future is in slightly more doubt with M3 being a rather established name in the business.
Here are two skins I have so far: