I've always had good luck with CTD. And CDNN. Great places to find oddball stuff and even mainstream. Love their catalogs too. Sometimes their prices are incredible, other times you can do better - just like anywhere else. For small orders shipping can seem high but you can wait for a shipping special.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek