A tip of the hat to The Rascal for pointing me to this URL for 80% complete AR-15 receiver kits and completion tools/jigs/instructions . This could prove useful if you live in a State that has registration requirements for “private party” gun purchases, but not for home built guns. Consult your State and local laws before ordering! You should also order through a drop box  to avoid a paper trail.
To U.S. readers: Resist the urge to drill an extra hole above the selector switch hole. With practice, you can squeeze your trigger finger very rapidly with just semi-auto. There is NO REASON to risk a five year stay at the Gray Bar Hotel and the permanent loss of your gun ownership and voting rights! I’m dead serious about this.
OBTW, I’m not a great fan of Mouse Guns . However, they do have their purposes, most notably for use by children, the elderly, and anyone confined to a wheelchair. Also keep in mind that AR-15 receivers can be used for more than just building a .223 AR-15. For example, the Ferret .50 kit  (.50 BMG single shot) requires NO FFL. Nor does the BRP Guns  MG-42 (semi-auto belt fed) kit. Both use AR-15 receivers. Parenthetically, a strange quirk of U.S. law makes the lower receiver on an AR legally the receiver, rather than the upper receiver. It is a good thing that Eugene Stoner set the precedent of stamping the serial numbers on the lowers of the first AR-10s, some 40+ years ago. The rest is history.