Congrats on your retirement and have fun on your build. I had alot of fun. And thanks for the brief history lesson.
I got my Mega receivers from Carrico'z. Other good places (when available) are Rainier Arms and Coleman Tyler.

http://www.rainierarms.com/

http://www.ar15web.com/categories.php

http://www.carricozarmory.com/lowers.htm

Be patient when looking for availability as Mega Machine Shop, an aerospace parts manufacturer, does not produce the AR recievers very quickly. Good luck!