Nordstrom's, Macy's, Neiman Marcus, or any of the big department stores with sales staff actually trained to measure clothes for fit is a good place to go. Just ask one of the sales assistants in the lingere department to help you. Otherwise, the smaller more specialized bridal shops should have someone on staff to help you. They often do in-house alterations etc. and would therefore know how to properly measure you.
I'd also recommend waiting a little longer to get sized for your clothes, so that you'll have a better idea of your size closer to the big day. If you will be nursing at the time of the wedding, you will more than likely be about the same size as you are now, unless you go on a serious diet/exercise plan.