Formal boutiques in boston?

I'm looking for a dress to wear to my prom at the end of May. I have already been through most of the major department stores (Bloomingdales, Saks, Neiman Marcus...) as well as BCBG, so I've hit all the typical prom-dress spots without any luck. What I'm looking for now is a boutique in Boston that sells the kinds of formal dresses one could wear to a prom. I'm not willing to spend anything more than $500 on it, so please don't recommend Valentino or something! Thanks!