Here's my favorite retailers:
www.airsoft1.com--probably the best place for buying clones, since they've got great prices, great customer service (CS), and some nice features, like choice of warranty, and the option to have them handpick your gun so you know it will work. No more lemons!
www.gunnerairsoft.com--Fung gets the clones before anyone else, and has solid prices. Don't let the high shipping prices fool you (about 60-70 on average per gun from HK), you'll be paying close to the same from anyone, and gunner is willing to try price matching. He just hired some new people, so expect to see his service getting faster.
www.airsoftgi.com--always reliable, one of the most popular retailers, although you may get a lemon since they don't check their guns.
www.nitroairsoft.com--a new seller, who got himself in over his head and had some problems, but is now recovering. Good prices, improving CS, and he does get new guns very quickly. And the always free shipping is a nice touch.
www.pointact.com--wholesalers, the best prices you can find on clones, just remember that they have a 150 dollar minimum order. Best place to find replacement gearboxes, batteries, and magazines.
www.ehobbyasia.com--an ebay seller, always good prices and CS on Hong Kong products. Great selection, and a good place to buy clones or upgrade parts, or cheap gear, or basically just about anything on a budget. They're a popular seller, and very few people have ever had problems with them.
www.airsplat.com--They just reworked their site. They've had some bad CS in the past if you get a lemon, but most people don't, and they have very good prices. Usually a good place to check out for whatever your needs.
www.evike.com--They usually have good prices, and like airsplat, they're very good when the gun isn't a lemon. But expect to have problems if the gun doesn't arrive in good condition. Some things to remember--Matrix is not an actual brand, it's a rebrand for all reasonably good clones coming out of China.
www.shortyusa.com--Great site with good CS and cheap prices, but only good for those just getting into airsoft. They excel at springers and LPEG's, but their prices for higher end guns are not very competitive. Always reliable, with fast shipping. I still do business with them.
www.airsoftatlanta.com--not always the best prices, but they have great selection and CS. Good place to look for gear or higher end guns.
www.redwolfairsoft.com--they have a middle of the ground reputation, but they do have some awesome custom guns that are worth checking out if you have some money to spend. Mixed reviews on these folks, so be wary. They are famous for inventing the Poor Man's Chrono--that is, the coke can FPS test.
How about getting upgrade parts? The big four, if we include
www.ehobbyasia.com, are:
www.dentrinity.com--US based retailer.
www.uncompany.com--overseas based, but the best selection of parts.
www.wgcshop.com--Hong Kong based, but also very good selection.
I also always liked checking out
www.gamepod.com, but they have terrible prices. I suggest you look around, and then buy what you want from someone else.
Anything else I missed? In my opinion, these are the best places to buy, and for the most part, if you're buying elsewhere, you're not getting the best value. To wit, I never shop at jungletoy, hobbytron, or Dick's Sporting goods, as I can always find better prices elsewhere. Feel free to add to the list, and tell us why you like the retailer!