http://www.dsflashcart.com (reasonable price, DHL will actually work, they have memberships here at GBAtemp, and they are really good with customer service)
http://www.card-nds.com (okay-ish price, but nice customer service. Responds quickly, and shipping reliable)
http://www.zhuzhuchina.com (takes Paypal, very nice people, reasonable shipping speeds and price)
http://www.realhotstuff.com (well-known for years now, fast shipping in the US)
http://www.realhotstuff.tk (Same as above, but ship from Hong Kong)
http://www.nds-card.com (having issues as of late apparently, but they reasonably price things, and had decent customer service up until now)
http://www.gamekool.com (Very nice people, been around for several years, one of my sponsors. Reasonably priced with good service)
http://www.gamezway.com (good service with a fair price. Shipping is decent enough, both express and with standard China Post)
I've dealt with, in some way or another, all of the above sites. I've had positive results come out of each one of them. Pick one site that fits your fancy, in terms of shipping time, cost, shipping method, reliability, etc., go with it, and just be patient. These things can take time to arrive.