So far I've been hearing good things about the following places in regards to finding a dress:
Dillards, Macy's, Group USA (perhaps more for the younger crowd), David's bridal (for something more traditional and toned-down), and other small boutique shops.
I've also found a new website called Vampel Dresses that seems to have a number of options. If your really on a budget don't neglect the vintage and thrift stores around town. Sometimes it is really amazing what people donate when they clean out their closets. TJ Maxx on occasion even has formal long dresses. Keep shopping and let us all know where great dresses are to be found!