zhuzhuchina is a completely reliably and trustworthy site.
I personally use realhotstuff.com most of the time, and they're good to go with if you live in the US and want your stuff pronto, because they're also US based. Downside is the carts tend to cost a buck or two more and you do have to pay shipping, but you're not going to be waiting a month and a half for the stuff.
nds-card.com is also a reliable site, and also is the official sponsor of GBAtemp, so your purchases here will support the forum. Shipping (iirc) is free, but again, it's coming from China so it's not going to be here in a matter of days.
As for card choice, consider the Supercard DSTWO as well. It's been time proven to be a solid piece, never having needed a hardware revision to continue working (usually just a kernel update, and occasionally a firmware flash). Compatibility with roms is near or at 100% due to having "clean" mode, which emulates an official cartridge while playing a rom but disables all cheats and RTS functionality (GREAT for new games with AP, though I don't foresee us having to worry about that much more), and "patch" mode which doesn't have 100% compatibility (until an EOS update is released to work with new roms), but allows you to use cheats and real time save and the like. It also has an onboard CPU that can have exclusive homebrew written for it, of note a SNES emulator and a GBA emulator as of right now, both being maintained by 'temp member Nebuleon. GBA one is a great thing on anything newer than a DS Lite.
Downside, however, is the price - they go for around $40. I have two of them though and my little brother has one, and they're freaking great. You also can update them (though it's not advised) on a console that has been too far updated to run it, good if you don't have a DS/Lite to use for updating. This is something that the R4i Gold 3DS can't do. If you've got a few extra bucks though or can handle saving just a little bit longer, I would
strongly recommend the DSTWO over anything.
Edit - some links:
Supercard DSTWO - NDS-Card
R4i Gold 3DS - NDS-Card
Supercard DSTWO - RealHotStuff
R4i Gold 3DS - RealHotStuff
Supercard DSTWO - ZhuZhuChina
R4i Gold 3DS - ZhuZhuChina