I didn’t have much luck finding apparel designed specifically for hunting when I lived in Boston. I think most people buy online or pass used stuff around the hunt club. The only consignment shop I know close to Boston is Horseman’s Exchange in Littleton - http://horsemans-exchange.com/
I’ve heard there’s a small consignment shop in Pembroke - Horsewares & Horseworn - http://www.riverwindfarm.com/horsewares.php
For a new jacket (you might find a regular hunt coat, but probably not a melton), check out the bargain sections of Dover and Smartpak, the closest stores to Boston. I don’t remember either selling hunt vests, but you might call ahead to see if they do now. If I were you, I’d post on Foxhunters Online specifically what you’re looking for and explain that you’ll be available to meet in Boston.