I'm teetering on that straight/plus borderline, and I've had surprisingly good luck at Group USA, which also sells online at
www.camillelavie.com. Many of their dresses go to a 24. I also recently bought a dress I loved from
www.asos.com, from a line called Chi Chi London. I just took a quick spin through the site and that brand seems to stop at a 20, but not sure whether that's all the time or whether they're just low on stock. The site has other plus options.
Nordstrom Rack is my go-to, though.