I visit the following and buy parts for my own builds:
bravocompanyusa.com
cmmginc.com
del-ton.com
cheaperthandirt.com
Copesdistributing.net
Stagarms.com
these are my favs for mags, bolt carrier groups, uppers and lowers, parts kits, ammo, and whatever else I need for ARs
del-ton is currently back-ordered for three months or more. CMMG is just as bad if not worse. Stag has yet to write me back. Most of bravo's inventory is gone. CTD is price-gouging. Cope's still had some AK and AR mags that I grabbed from them a few days ago.
This is a real bad time to be shopping for anything AK, AR, Glock, etc.-related. Good luck.