My favorite is MidwayUSA. They carry a large variety of accessories from many manufacturers and the site tells you when you're looking at an item if they have it in stock. If they have it in they ship pretty fast. They also participate in NRA Roundup so you can donate when you place your order if you'd like to.
Here is the list of sites that I visit for accessories:
Cheaper than Dirt
*These vendors carry some good stuff, but you need to look at what you're buying (check reviews), as some of the components or accessories may be more applicable for toys or AirSoft than an actual .223/5.56 AR.
It really depends on what accessory you are looking for. I buy from several different sites because I usually look for the lowest price. I rarely buy from Ebay, unless I am looking for something lightly used.
GrabAGun.com and Buds Gun Shop usually have the lowest prices (with shipping) by a wide margin, granted they carry what you are looking for. Some other sites that have unique and high quality accessories are La Rue (optics mounts), Impact Weapons Components (sling, bipod, and light mounts), SWFA (optics and mounts), Vikings Tactics (sling components) etc.
GG&G, Midwest Industries, Bravo Co., Magpul, Knights Armament, Troy, etc all make and sell quality components. I probably own at least one accessory made by almost all of them. Many of their accessories can be purchased direct or through other retailers.
Unfortunately Google no longer shows "shopping" results in their searches, so you can't easily compare prices. If you know what you are looking for you just have to put in the extra effort of manually going from site to site to find the best price.