I like many others have used these site sponsors and have had no issues what-so-ever
OSTS, CTCS, Armtac, Questar, Sask Gun Works & SRFC.
As noted, having the right tools also helps. Upper/Lower Vise Blocks (Spikes, Magpul etc), Roll Pin Punches, Armorers Multi-Tool (DPMS) go a long way to making things easier during the course of the build.
Sometimes sourcing the actual parts is the maddening fun of it all, so don't bank on just one supplier. Utilise the EE when you can and you'll find deals if you have the patience and understand current prices.
Good luck!