My favorite place to shop online for dress clothes is Dress Barn. Despite its name, I think the clothes are very modern and cute. Everything I have purchased has fit well, even the pants. I also like that while the clothes look young and modern, they work well for a woman; the pants are higher-waisted and are made to hide mommy-tummy. Most of the dresses run around $40.00 but very often there are online coupons for $10.00 of 2 dresses or free shipping. They also have great customer service.
Otherwise, most of the clothes I buy come from thrift stores. I like Uptown Cheapskate and Plato's Closet; they are the type of stores you can sell your gently used clothing to. While you do have to search through tons of inventory and try on a lot as everything has a different fit, I find a lot of nice clothes, including shoes, in the $4-$10 range. I also like Savers and Deseret Industries. In fact, I just bought 3 skirts, 2 shirts and 2 cardigans for $30.00 this week and all look brand new and were name brand clothes from DI. A lot of wealthier people in my area seem to donate super nice items. Sometimes Target will have cute, cheap clothes as well but I find the fit is really hit or miss, so I would never buy online from them.