Well I am plus sized myself so when I went looking I went to macys. They have such amazing dresses for plus size teens.
You might have a hard time finding it in local stores, but many online retailers specialize in plus sizes. Try www.buycostumes.com/PlusSize or www.plussizecostume.com
Here is a link to a website that offers a variety of different pretty homecoming dresses under $100 dollars. http://www.homecomingdressesunder100.com/
Ebay is a famous online store that sell various goods including high quality velvet dresses with customer's choice of price after each auction. Good and inexpensive velvet dresses may be also available during discount period in several clothes stores.
Lulu's, Gojane, and Amazon are good places online to find inexpensive juniors dresses. Some local stores may also provide these products at low prices.
Try going to a department store. A lot of department stores will have much cheaper prices then stores that sell only prom or event dresses. If you still want more options, try going to an auction site or putting an ad in the news paper.
There are various stores that sell inexpensive prom dresses online, like milanoo and jjshouse. Also, one could look in less expensive clothing stores like Forever 21 and Agaci Too for trendy dresses. JC Penney, Dillards and Macys also run really good sales on prom dresses.
Macy's JC Pennies Kohls DEBS,or milanoo.com has very inexpensive dresses.
Century 21 sells really inexpensive dresses for teenagers in NY. They carry huge discounts on designer brands so your teen will be able to find fancy stuff for cheap. It is located on Cortlandt St.
stores
Thrift stores like Goodwill or Salvation Army are the best place to find inexpensive dresses that won't be worn very often. You need to check them often in order to find what you want at the best price.
Blue dresses can be found in a variety of price ranges at a number of stores. A few stores that sell blue dresses are Target, Express, and Nordstrom.
There are so many prom dress stores sell dresses for prom online, and you may hard to find inexpensive prom dresses under 200, they have short cocktail gowns, semi formal dresses, white dresses for graduation dresses and prom dresses 2012 in all sizes and shapes from petites to plus, tutu, baby doll to mermaid styles. Payment through paypal, free shipping in about 15-25 days and good service.