Self-designed carbon fiber highracer, BikesDirect Kilo WT5, Pacific Cycles Carryme, Dahon Boardwalk with custom Sturmey Archer wheelset
I recommend thorusa.com
Not the cheapest, but you're already getting a good deal because you're buying a Dahon and the extra quality control and support from Thor is worth it. Lots of Dahon dealers don't know and don't care how to properly set their bikes up, but Thorsten will take care of you to make sure you get your money's worth out of the bike (you see, it's a labor of love for him which means you get expert advice which you didn't pay for...he even wastes his time posting on this forum like us other folding bike geeks, except he knows more about Dahons than anyone else).
Trust me, the shop that's selling for $519 with free shipping is my primary bike shop (located about 1.5 miles from my house). While they're nice and more knowledgeable than most bike shops, I only shop there because they have a big selection of parts in stock, but they really don't know much about folding bikes (or bikes in general) and won't be able to help you if/when there is a problem (as there tends to be with Dahons).
Granted if you're a novice you might not notice potential problems or ride hard enough to break potentially misadjusted parts; So you might not care about having everything tip top. But if you plan on riding lots and hard then the extra $80 paid to Thor is well worth his expert knowledge and care (unless, of course, you're already a Dahon expert...then you might not need Thor's expertise).