Stomp has a funky atmosphere and stocks most everything you need, prices are comparable to the other homebrew stores, but my personal preference is The Brew Hut, part of Dry Dock Brewing Co off of Hampden. Huge grain room, some of the thing they buy in bulk are less expensive (LME, for example) and the yeast/hops selection is large and fresh. Bonus, make your purchases, then hang out for a pint or two in the most excellent Dry Dock tasting room. When I was there last month they had 16 beers on tap, all their own. Fantastic!