Need a dress for formal occasion in August?

I am a young woman, age 28 and I have been searching for formal attire for a banquet that I am attending in the month of August. This has been no easy task because my dress size is a size 18 and most of the things I found in my size were not acceptable. If anyone knows a place that sells stylish (not frumpy) formal or evening clothing at reasonable prices I would really appreciate the help.