As for the best card, it's debatable. The DSTwo in terms of features is the best, as it has a CPU in the card that lets it run GBA emulators and SNES emulators at (mostly) full speeds. It's the more expensive of the DS Flashcards. Then there's the choice between the Acekard 2i and one of the many R4i clones. I personally would choose the AK2i, seeing as I've had one since launch and it has yet to fail me.
I got mine from an online shop (one of those shown in the actual .cn website).
This website: http://www.digimartz.com seems to be an official retailer according to their website, also, it's located within the UK (although the shipping will most likely come from china or some other country as per usual).
Once before,GBA forum hold a "favorite flashcart poll",from the results,the three popular cards specially welcomed by us are dstwo,ak2i and r4i gold 3ds. If you are interested in r4i series,then r4i gold 3ds is your choice, which with famous wood firmware,quick firmware update,works perfectly on 3ds and dsi latest firmware. You can come to r4ids.cn distributor list ,or google it,or just go to shoptemp,you will find tons of stores .
They listed "R4iDS.CN", but the picture says "WW.R4i-SDHC.COM.AU", the supplied photos are of a different product. They listed "R4iDS.CN" on the wrong cart (again, the product you listed is "R4i Gold Revolution", not "R4i Gold".
The e-mail says the product is out of stock and not produced any longer, which is not true for the actual R4i Gold with wood support.
Just because it's a recommended site doesn't mean it only carries the good carts. Every flash cart site carries the clones because it still makes them money, and they simply list up the descriptions and such that the carts send them.
Obviously, the reply from nds-card is incorrect and it might mislead to some new users, the r4i gold 3ds from r4ids.cn is still producing and selling in every site. And another, the pic they listed in their site is the older version, r4ids.cn has changed the package to this new one.