I see you're in Chelsea... when feeding my Saiga 7.62 with Mass food, i've found the best deals at Collectors in Stoneham. Pretty much anywhere in NH if you are willing to make the trip will beat that a bit though, especially after sales tax. As others have said, MFLR (manchester) or Riley's (Hooksett) can't be beat, especially in bulk.
Edit: Just went back and some prices mentioned were for Yugo. Yugo is great shooting stuff, and brass... but its corrosive so plan on douching the shit out of your gun after a trip. Silver/Brown bear stuff works great in my Saiga, and is non-corrosive.