I've actually had fairly decent luck the last few years (I'm an 18-20, usually). I do Renaissance faires and the stuff I wear to faire is pretty adaptable for Halloween. Many vendors sell plus sizes and the clothing isn't always obviously "rennie". One year, I went with friends as the Sanderson sisters (Hocus Pocus - I was Mary); another year, with a corset, ball gown skirt and jacket, I went as a "dominatrix" vampire; last year, pretty simple as a sexy witch in my black velvet dress from
Faire Pair tights & a hat from Target - I love this dress - it's very versatile! I can add lots of accessories to it for a costume or wear it plain to a party. I'll absolutely agree that buying a ready-made costume at a shop hasn't been easy, but I've been able to find some good ideas.