S.W.
Until recently (now that I've lost 60 pounds and can shop in regular stores), I could only shop in plus-size stores. At my heaviest, I was a size 20 (almost 22). My main clothing source was Lane Bryant. I still have a credit card for this store because I can still buy my underwear and bras there, but the rest of the clothes are too big for me (mainly the arm & neck openings are too big or the crotch of the pants are too loose). I suggest you get on their mailing list, even if you aren't going to sign up for their credit card because they send you coupons constantly and to be honest, I don't like paying full price for their clothes. They are great clothes; I just think their regular prices are a bit steep.
I also had good luck at Dress Barn a few times, especially for cute dresses, which can be hard to find in plus sizes. Avenue sometimes has cute clothes, but some seasons they are just awful (although I also felt that way about Lane Bryant). I also suggest trying different Lane Bryant stores because their inventory will differ greatly based on the buyer for that store. Some stores REFUSE to carry dresses; only some stores carry bathing suits; some stores also carry shoes, while others don't. I knew that depending on what I was looking for, which store I should go to for that particular item.