Walmart
Deb has a plus sized formal section. Check them out at depshops.com or see if there is a store in your area. They have a pretty good selection at very reasonable prices.
There are some store that specialize in plus size prom dresses. Most store have a variety of prom dresses including plus sizes. You will find a great selection of plus size prom dressesat Macys. You can also order a dress online at sydneyscloset.com.
There are companies that have good deals on plus size prom dresses. One store that has good deals on the dresses is Macy's. Macy's regularly gives out coupons to customers.
For fashionable plus size dresses, you can always check trendy clothes store and sites such as H&M, Forever 21, Urban Outfitters, as well as Abercrombie and Fitch, or Hollister.
Unfortunately "off the rack" stores generally don't carry plus size dresses like this, although if you look for a plus size store that specializes in larger clothing you can find something better tailored to your shape/figure.
For lovely plus sized summer dresses Lane Bryant, Torrid, and The Avenue are wonderful options. I also go to Ross and Marshalls to get department store dresses for half the price.
Nordstrom and Talbots located in Columbus, Ohio offers great selections in plus size dresses. Keep in mind that Go Figure Consignment carries dresses in plus sizes that will fit any budget.
There are a number of plus size dresses online. Macy's sell plus size dresses normally for sale online and in stores, although their prices are still quite high.
You can find plus sized dresses for a child by going to Jeeny Beans, Sears, Bonnie Jeans, and Old Navy, Gap, and Elegant Plus. You can shop on amazon.com.
There are actually a lot of plus size stores around nowadays, you can find them at most malls or shopping centers. How much the clothes cost will vary from store to store. Here's a website link to a plus sized store that has good prices on a variety of dresses. http://www.avenue.com/
As long as it's in the summer or the spring you can find plus size dresses on the website. They offer a wide range of plus size dresses and petite sizes as well.