I would start off at:
Sturmgewehr Assault Rifle and NFA Forums - On the New Server
Tom Bowers' Politically Incorrect Machine Gun Pages has some good info and decent classifieds as well.
ARF has some pretty active NFA classifieds: NFA Firearms & Parts - AR15.COM
ARF also has good subsections in their Armory section: Armory - AR15.COM
There was also a gunfinder page where you would type in whatever gun you wanted and it would search the internet for you but I can't find that page anymore. Maybe you can use your google button better than me.
These should get you started and keep you occupied for awhile.