I'll be the first to say that I think it's silly that most of the sample dresses are the smaller sizes. If the average woman in the US is a size 12, brides are getting married older (not 16 or 1, and the dresses run two sizes smaller - why don't they order sample sizes in 14s or 16?! That has really irritated me the entire time.
I'd call back to the shop, asked to speak to a manager and let them know about your experience. Then, find a shop with a really fantastic reputation in your area and call to see if they have that dress or can get a sample that would be about your size. Sometimes if you ask nicely they will order samples in other sizes.