David's Bridal has some you can look at through their website, though I think the store has to special order the plus sizes, at least that's what they told me once upon a time. If you find one you like on the website just be sure to leave enough time for them to get it in and for you to have enough time to find something else if you don't like it after all.
http://www.davidsbridal.com/index.jsp
I just got a formal at Dillard's, if you have those where you are. A lot of the stuff looks like something my mom would wear, yes, but they have quite a few younger looking dresses too. Don't try the website though, as the website doesn't have a very good selection. If you have a Dillard's in town, go look there and try more than one store if there are some around you.
Nordstrom's and Macy's also has some plus size formal stuff, though I've never looked for myself, I've only been told.
http://www.nikibynikilivas.com/ She has some nice dresses and many of them come in plus sizes. This is the designer that made the dress I just bought at Dillards, but my dress isn't on the site for some reason. Anywho, if you find a dress you like there, that site has a link of what stores near you carry her lines.
http://www.groupusa.com/ Most of their dresses go up to a 22, some to a 24, but be warned, they are one size smaller than they claim (AKA: a 24 fits more like a 22, 22 is more like a 20, etc etc). If you have one of these near you, it's worth a shot though, they are having great sales right now. There is a store locator on the website. You can browse the site, but I do reccomend just going in if you can, there are more styles in the store than on the site.
Hope these are helpful, if I think of any more I'll post again.