For suppliers I used Classic firearms, lower parts kit and stock/buffer tube kit. 80% lower was a raw piece, either American made Tactical or a new frontier piece. Sourced handguard from DSG, also eBay has suppliers for handguards as low as 35$. Am getting barrels from Bear Creek Arsenal. Barrel nut either comes with free float handguard or from Classic Firearms. Muzzle brakes are another eBay or Amazon purchase. Didn’t put in ejection port doors or bolt advance. Mine are 80% raw upper and lower, and just spray bomb. Bolts, I have seen for $100 at Brownells. If using a standard carbine kit you will need a buffer spacer, New Frontier Armory. Sights have been a clearance TruGlo from Scheels, $60. If you don’t want a spacer in buffer tube then get buffer for 9mm use. Brownells/Classic/New Frontier. Prices change constantly and depending on your build goals. But shop around.