Deb stores
There are many websites that have a wide variety of strapless dresses for sale. Nordstrom has strapless dresses online, and also many other websites that have multiple brands together in one place so you do not have to skip around.
Nordstrom's (not Norstrom's) Online site carries many strapless dresses for purchase. The easiest way to find them is to use the search box and type in "strapless dress" or "strapless dresses".
Abercrombie Kids
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Strapless dresses are super cute to wear. Hope the following link will give you some helpful information on where to find a strapless dress. http://www.polyvore.com/where_can_find_strapless_dress/advice.show?id=112451
I would go check out Longitude swimwear. They have numerous styles, colors; strap, strapless, dresses, etc. All of this a for a reasonable price as well.
if you're open to buying something online, then that's a good place to look! forever 21 also carries various peplum dresses, and you might find one at H&M or papaya-- really, any store that has a selection of teen / young adult clothing would probably have some!
Donnaricco.com is a great place to find dresses for any season, they have new summer and fall dresses daily. They do have petite sizes as well as regular sizes, and you can even search dresses by occasion as well!
You'll most likely find nice things at macy's, gap, banana republic, or zara's... They have a sophisticated "modest" look
You can find a strapless prom dress for cheap from the Salvation Army, Volunteers of America, Amazon.com, Good Will, and other secondhand and thrift stores. You can also find one a plato's closet.
Short is definitely trendy at the moment. You can find trendy plus size dresses on the following site: http://www.torrid.com/torrid/Homepage.jsp.
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.