I went and scouted today, and based on what I saw you're talking about Dauphine then. Dress to Kill is in an awkward position, a block or two away from Canal St. and parking is hard to find. I only drove by for that reason.
It turns out that the shop I was thinking about only sells masquerade masks now. I didn't really see many other stores in the FQ. I know Queen Fashions (
www.queenfashions.com) is a local shop, but they ship from an address in Metairie and I didn't see a shop downtown (their phone number has a downtown prefix), so it might be worth it to give them a call.
If you're gonna go the Payless route, the one a block past the Ritz on Canal is the one on Canal that sells a size 13. Most other shops in Orleans go up to a 13 as well. I have yet to find a size 13 in any other store here yet.