If you can travel, go to a Nordstrom Rack or an Off Fifth Saks Fifth Avenue Outlet in a large city suburb (I'm thinking Chicago, perhaps.)
They will usually have quality plus-size clothing, such as Ellen Tracy, Dana Buchman, Lauren Ralph Lauren, Jones New York, Due Per Due, Talora and Classiques, et al. If you can get to a Bloomingdale's or a Macy's, they will have INC and Alfani Woman. If you can get to a Loehmann's, they have Marina Rinaldi (which is very pricey in stores but about 80 percent to 90 percent off)
I have to tell you, however, that bridge and better plus-size clothing is generally designed and marketed for people 30 and older, career women. Because of that, I can't help you with junior plus sizes. BTW, I'm a 37-year-old "tweener" who wears plus on my bottom.
Also, for the Web try:
www.alight.com
www.ullapopken.com (sizes 12-34)
www.kiyonna.com
www.avenue.com (Used to be Lerner Woman, not bad for mass clothing)