Honestly, if you have a David's Bridal around, give them a shot. They sell a wide range of sizes and they're reasonably priced. You might even catch a good sale.

When I got married, I called multiple smaller boutiques and shops in my area and asked what size they went up and had little luck finding shops that sold over a size 16. It was disheartening. The only plus size boutique around here was way out of my price range. I went to David's bridal and was fearful that they wouldn't sell my size, but I was really impressed by the selection and I found a beautiful corset back dress on the clearance rack in my size for $300.