I've "built" 5 different AR's in the past 2 yrs. Assembled is more correct really when it comes to AR's.
Bought/sourced parts from various CDN companies as well as a boatload of stuff off of the exchange.
I didn't even bother trying to order/buy from the US...even if it appears possible.
A...just looking for grief and B....SUPPORT CDN firearms companies that miraculously conduct business in this country...especially in AR and other evil stuff
Be aware that you may not necessarily either build a better or less expensive rifle compared to buying one complete. You may also experience some parts compatibility issues...usually nothing serious but there can be some issues.
What you will experience is a learning curve on the AR that you would otherwise not gain...how it works/ass/disassembly/potential and actual failure and malfunction areas...the experience of putting together your own version of what you want your AR to be...and FUN...its like adult gunlover lego!!
Oh and I might add that my bone stock Norc AR runs and shoots as well as rifles costing 10x as much.... just sayin.
There are areas where some high quality components do perform better.....a good quality plated BCG makes cleaning life easier and the rifle run smoother/more reliably when dirty or running long strings of shooting. A lot of the other components will make lil to no difference...aside from cool factor and durability.
Be realistic about what yer going to do with it.....
And further ..just do it....buying/building/shooting AR's is tantamount to a voodoo doll for liberal's