http://www.wekeepyoucycling.com always ships same day, and often has some great prices. They are my favorite for the items they carry at good prices.
Amazon, of course. I always check there, and if the price is close will buy from them and do the free shipping thing.
http://www.cambriabike.com/ is a good one.
And of course
http://www.probikekit.com/us/ when they have a discount going. Just have to plan for the long ship times.