POst in the AR-10/ar-180 subsection. IIRC, there were 2 or 3 owners that had actual full auto and a bunch of ar-180 owners that can at least point you to the right supplier for parts. I had an old costa mesa semi auto back int he day which i loved dearly to death. Being a broke college student at the time, I couldn't afford to keep her so I had to sell her. Dealer had a pre sample full auto, and mentioned it was a smooth shooter but parts were either hit or miss. That was in 2001, I can only imagine parts are only more hit or miss. I'd post up on subguns and sturmgewehr to see if you can get more feedback and guidance.
Best of luck, I still miss that solid folding stock that would beat the living daylights out of anyone.
Oh and numrich has some random parts left in case you need any.
http://www.gunpartscorp.com/Manufacturers/Armalite-33128/AR18-34714.htm?page=3