Anthropologie is usually my first thought when someone asks me about dresses. A close second place goes to ASOS. The trouble with ASOS is the large amount of items listed online. It can take you hours to scroll through every item, even after you filter for your size and price range. If you're willing to put in some time for the search, ASOS has some hidden gems.
If you don’t have time, and are looking for a great deal be sure you click through the options linked below using the arrow to the right of the photos. I've taken the time to search, and dig, and scroll (and scroll and scroll and scroll) to find great deals. My favorite might be this Club Monaco look alike that was available at Neiman Marcus last fall.