Depends on what you are looking for, some places have better deals on complete rifles, uppers, lowers, etc... some places have better deals on small parts.
Primary Arms is good, Palmetto State Armory (have to pay sales tax and high priced/very slow shipping), Rainier Arms, JoeBob Outfitters, Operation Parts, Midway USA and Brownells are decent, and AIM Surplus.
You can get good quality barrels at Bear Creek Arsenal, good prices, right now they are low on inventory, they also carry other parts as well. I've got a local FFL that carries a lot of AR parts, but he's out of lowers right now, been a run on them lately.
Ballisticdeals.com is a new one I found. Best prices ive seen, primary arms and psa. I prefer primary arms, faster processing and shipping. Psa has good deals big can be SUPER slow and I'm impatiant.
With PSA, I've found that if you order normal stuff, the shipping is very fast. If you're ordering a complete upper or lower, it's considerably slower because they build them to order. But still not bad.
I bought one of their 18" Varmint uppers 1 in 7" twist barrels and had them flute it to save some weight. It runs like a sewing machine. I also put a GIESSELE SSA-E trigger and a Bushnell AR Tactical 4.5-18x40 scope on top.
Here is the first 5 shot target I shot at 100 yds. with left over bullets for another rifle.
There are a ton of places to get accessories from, you can get quite a few things from Amazon believe it or not, at very good prices I might add. I've been checking various websites for a while now , comparing prices and quality of items, a lot of cheaply made junk parts out there as well. I've just picked up two lowers and will be building one using the least expensive but good quality parts I can find, it gets confusing sometimes trying to sort it all out ! it pays to check multiple sites though, as there is a huge difference in price for the same items a lot of times from one to another.
PSA, Primary Arms, Delta Team Tactical, Midway, Brownells, Joe-Bob outfitters, to name a few. When I want something I search for it until I find the lowest price, or if looking for multiple items I pick the site that will result in the lowest total cost after shipping. I'll pay a few bucks more for an item if I can combine shipping with the rest of my list as opposed to paying additional shipping to order it from another site.
I have always been spoiled. I always try to get the most accurate gun I can. You probably can get a better price, but you can not get a more accurate one than from WOA.
Delta Team Tactical has been running some real good sales lately, both on barrels and on build kits (minus lowers) . I've bought two barrels from them in the last month, name brand barrels cheaper than I could find elsewhere.