I had favorites when I was younger and didn't dress like a secretary all the time, but most of them have shut down by now (or at least moved out of the area).
Honestly though, most of my clothes came from either Target or Avenue, sometimes Torrid. Avenue has some painfully ugly stuff but they're cheap, so if you sift through the ugly, you can find some gems. ChicStar had some really neat stuff last time I checked them out, but they also have some whoa-that-is-way-too-weird kind of stuff going on as well so you have to kind of search around for what's good versus what will make you look like a weeb/mallgoth. I always liked H&M as well but whenever I went in to try things on it seemed they had that same issue with designing for flat chests so I gave up on shopping there. It might've changed in the recent years - I hope it did - but eh.
Thrifting is always hit or miss. I got a really sweet button-down for free (!!!) and was intent on turning it into one of those tacky vests with the patches and the studs, but it's so friggin stiff for some reason - can't do anything to it until I give it a salt bath.
Thrift stores, or the internet. I would literally buy everything off ModCloth if I had the money. Anywhere that sells nice clothes for fat people though are over priced. >.<
Have you tried Torrid at all? I think they started as an offshoot for Hot Topic but they carry some nice stuff now. Some of it can be a little pricey, depending, but mostly they're pretty reasonable I think. You might like ChicStar too I think!
I order all my stuff online too but haven't looked at ModCloth. Now I have an excuse 8D