Maybe from a site on the TOR network, not legally in the States unless they're shipping to an FFL and you have the required clear background to pick it up. Internet black markets won't self impose restrictions.

The receiver is the most restricted part of a gun, it's where the round is chambered. You can have all the plastic lowers for ARs in a basket all day long...but a full metal working receiver capable of withstanding the immense pressure of a round being fired are heavily restricted and are considered the gun. The lower is really just a toy.

This is a lower (it's just a stock, a trigger assembly, a hammer). The only thing that is being printed is something like this.

Plastic parts in the trigger area, that have enough pressure to send the hammer forward with enough force to set off a primer really don't exist, not in the small confined space (they'd be big and bulky).

Don't be fooled by the media hype that people are printing entire guns. Use logic. It's impossible to do it. Who'd want to use a gun that could blow up in their face? Not I. Besides, plastic is too soft to get an appropriate twist on a bullet for it to do anything more than drop 10 feet from the barrel because the parts aren't precise enough to hold the pressure, aren't machined to exacting standards...the list goes on. It'll stick to metal over plastic, thanks.

I have a feeling you're trolling though.