Re: AR 15 build
Whever you can find them. Giving the rapidly oncoming assault weapon ban (to save Mexico), I'm not sure I'd try a build. You would do better to "lurk" outside a gunshow and make a private citizen buy from someone coming in to sell one. (no paper trail) Then get your large diameter plastic pipe........
Once you have a complete one you can buy uppers (so far) without any paperwork, so you can have anything from a 22 RF to a 50 Beowulf.
This new AG is really scary!