The Bushmaster (Carbon-15) 9mm Ars that I have seen have had plastic mags.
In a bygone era Questar used to have metal 5/32 converted UZI mags. I had a few, but I got rid of them because they were like 80% functional. They did not lock back on the last round, and they jammed sometimes.
My understanding is that "Colt" mags were converted UZI mags, up until supply got scarce - and then C-products started making them.
The first round of C-Products 10-round steel mags were so-so. Depends on the gun. The second round (orange followers) were good.
The best, in my view, are the plastic Pro-mag mags. There are 5/32 and 10/32 "pistol" marked mags (with the RRA logo, again another thing done by Questar in a bygone era).
As far as I can tell, the only mags that are available at all are the steel 10-rounders from Questar, or mags pinned to 5 rounds that are brought in with the rifles themselves or by other importers.
Don't bother asking Questar about UZi mags or 10/32 plastic mags, all of that is long, long gone. What they have is what is on their website.
I think Wolverine had some of the 5/32 Pro-mag plastics for awhile, but they are probably long since sold out.
9mm AR-15 can be frustrating to get up and running. You need to have your gun set up for your mags, and mags that work with the gun. I'[m just glad that I have my gun and all the mags set up and working 100% reliable.

I had my struggles in the beginning though.
http://www.youtube.com/watch?v=8Bj4-lluJOs