Below are some of the websites I've used to piece together my AR15. I either ordered from them or learning something from them. Or just checked them out during the building phase of my AR when I just had just pieced together lower receive with only a Grip from the DPMS lower receiver kit anxiously waiting the money to get some new parts.
www.bravocompanyusa.com
www.midwayusa.com
www.brownels.com
www.spikestactical.com
www.rbprecision.com
www.ar15outfitters.com
www.combathunting.com
www.PrimaryArms.com
www.triadtactical.com
www.ustacticalsupply.com
www.triadtactical.com
www.demontactical.com
www.longhorntactical.com