Depends on what your looking for. Here some I've used in addition to those mentioned:
Del-Ton, Inc. A complete line of AR-15 Rifles, AR 15 Parts and AR15 Accessories.
Quality optics and accessories from Leupold, Bushnell, Burris, Nikon and more!
Ammunition, Magazines, Rifle Parts and Accessories
Bravo Company USA, Inc. AR-15, M16, M4 Tactical Gear, Parts, Accessories.
Centerfire Systems, Inc. - Your Home For Ammo, Firearms, Magazines, and Gun Accessories
CDNN Sports, Inc.
Cheaper Than Dirt - America's Ultimate Shooting Sports Discounter
MidwayUSA — Shooting Supplies, Reloading, Gunsmithing, Hunting, Ammunition, Gun Parts & Rifle Scopes
Global Tactical - Tactical Rifle Parts & Accessories
LaRue Tactical Home Page
http://jtdistributing.net/index.html