Mother Of The Groom

My son is about to get engaged and I have FMS which makes it very difficult to do any kind of exercise as some times even walking or any kind of movement can be quite painful. I need to lose weight and have been trying for quite some time anyway. Now, with him about to become engaged, if I`m not able to lose as much as I need to, where are good places in the Atlanta area to find reasonable priced dresses for (more than likely) an afternoon wedding, for about a size 18-20 sized woman? Any help would be greatly appreciated. Thanks in advance.