What I do is watch for sales across as many sellers as possible. If you're diligent you can piece together your dream AR at "sale" prices. Watch the Monkey as several here post notices of good deals out there. Just because a barrel is on sale really cheap but has an A post sight is no reason not to get it. It's removable and someone somewhere will have your preferred gas block on sale. The mil front sight can be used or traded later for the next build. I should add that if you look at your A post sight you'll notice that with a hacksaw, file, and sand paper you now have a low profile gas block and still retain the ability to mount your own custom chainsaw bayonet!
Find a gun forum that covers your area, and buy parts face to face using cash. A serialized (non 80%) lower is the only new part you would have to go through an FFL. Also, can't you still do direct face to face sales in your state without an FFL? Before Oregon passed it's law requiring FFL's a few years ago (legislatively as an EMERGENCY bill to fix a problem that didn't exist) it was common to exchange in the parking lot of a sporting goods store.
@3M-TA3 "Also, can't you still do direct face to face sales in your state without an FFL?" Yes, I am pretty sure we can but I would have to double check it. I think all that's required is for you to sign a Bill of Sale which is why a lot of folks around here sell their guns as consignment in stores so it is on record if it every comes back to bite them as the store handles the sale in a FFL manner - meaning - the firearm is transferred over to the store and they transfer it to the individual who purchases it, taking a commission of course. I know that is how I sold my Kahr and my Remington 887 shotgun. Nevertheless, one sees firearms for sale in the local paper all the time. I will look into it.