these are the main brands i have used on 3 of my builds
Lower: Stag/CMT
Upper: Stag/CMT
LPK: Stag/ CMT
Stock: Magpul (pick one, i have an m93b and two CTR, love them both)
Barrel: Stag/CMT (20" and 16") or CMMG (16")
FF Rail: Daniel Defense or Larue (i am a firm believer in you get what you pay for)
BUIS: DPMS mangonel, ARMS, GG&G
Optic: Eotech
as for sources, just click on the industry link and start going through the dealers till you find the price you like. Good luck. also, keep an eye on the EE for deals that come along, its worth it to your pocketbook.