How would one go about buying a non-compliant barrel with the intention to modify it to be compliant? One that is threaded, for example, or that comes with a flash suppressor that could be removed. Are the options limited to local stock or already compliant parts online? 

You can buy a bare threaded barrel from almost any online retailer, no problem.  Just make sure whatever muzzle device you buy separately is compliant, and have it permanently attached, ie pinned and welded.  You could also forego a muzzle device altogether and get a non threaded barrel, or get a threaded barrel and install a simple thread/muzzle protector. 

17 hours ago, Scorpio64 said:

You can buy a bare threaded barrel from almost any online retailer, no problem.  Just make sure whatever muzzle device you buy separately is compliant, and have it permanently attached, ie pinned and welded.  You could also forego a muzzle device altogether and get a non threaded barrel, or get a threaded barrel and install a simple thread/muzzle protector. 

Ah, I see now that currently non-compliant parts can in face be shipped! There are some limitations though, which I don't know if they're across the board and also illegal to ship or just individual company policies. 

This is Palmetto State Armory as an example:
No Complete AR / AK Receivers
Non-compliant AR and AK rifles will not be shipped

So I guess buying an individual barrel, upper receiver, etc., is the way to do it? 

I'm guessing is someone built a compliant upper it could be sent directly to a personal address.

EDIT: I just did some testing at PSA and it looks like they won't even ship a complete lower to an FFL, but they will ship a compete not fully compliant upper to a personal address. They will even ship an A2 flash hider, but that's a paper weight. Then it's off to pin and weld. (Does a thread protector also need to be pinned and welded?)

Yeah, unfortunately doesn’t look like PSA delivers complete lowers to NJ

5 hours ago, Newbcake said:

Yeah, unfortunately doesn’t look like PSA delivers complete lowers to NJ

At least the lower is easy enough to assemble at home. Pin and weld, not so much. Which actually makes it more strange. The lower is only required to have the stock fixed in place which should be easy to accomplish.