I personally don't care for David's Bridal but they are the cheapest option around for bridesmaids dresses that come in a wide variety of colors and styles. They have 3 locations in the Atlanta area.
Alfred Angelo in Dunwoody is another good, cheap option with tons of styles and colors in lots of different sizes.
We purchased our bridesmaids dresses through Wedding Angels in Roswell. My bridesmaids got a slight discount because I had purchased my wedding gown there (something you may want to check in to if you've already purchased your dress from a salon). Personally, I prefer going to a salon because of the atmosphere and more personalized attention...but you may end up paying more per dress than you would at a discount/mass produced store like David's. Wedding Angels carries several bridesmaids dress lines and they have tons of color swatches available if you find a style you like.