My wife has a Cleopatra Capri suspender belt which she likes very much (as do I!) as it's both comfortable and pretty. You can see it on the manufacturer's website at:
http://www.cleopatralingerie.com/capri6.html
This is a wide belt with 6 straps and metal clips, which is a big advantage in terms of stability. My wife regularly wears this suspender belt out-and-about, and she's never felt any concern that her stockings are insecure.
From the packaging, the belt is available in black, white, bronze and red, and its "large" size covers a range of 32" to 38". I'm not quite sure how this corresponds to dress sizes, though. Is that a possibility?