I'm not too fond of their policies, but otherwise I've always had pleasant experiences with Old Glory/State Line in Mason, NH, I've made a few excellent purchases there. For a place a little bit more west of Mason, you can't beat Elise and Ty at American Weapon Solutions for customer service, AR's, AR parts, and for orders or transfers.
Collectible Arms / 619 Guns and Ammo is good to go in my book as well if you're talking a bit further East.