I LOVE thrift stores; I spent all day yesterday in them, and then came home and was leafing through my closet and, a few dresses aside, couldn't find any clothes that had come from anywhere other than a thrift store. Brand name, nearly new, averaging $6 a piece. Totally your friend.
The Salvation Army, in my experience, is really bad about organizing by size; even within the plus size, if they have a section (and they usually do), you'd find the whole range of plus size clothes jumbled together--really, really annoying to leaf through looking for the magic intersection of one that both is attractive AND your size.
You should look for a Savers near you:
http://www.savers.com/Find-A-Store.aspx
They're usually huge, clean, and really well-organized. And look for Goodwills near-ish to where rich people live, lol.