Harris Cyclery (of Sheldon Brown fame)
http://www.sheldonbrown.com/harris/index.html
Probably the fastest turnaround time I've experienced. I put an order in the morning, got the tracking number in the evening. I asked for some cable crimps, and they threw in a bunch of 'em for free. Nothing big, but it's a nice little detail.
If I can find it on Harris Cyclery's website, I'll buy it there.