What Reaper said. Tripp's research will do extra mags, in either hard chrome or their polymer coat for $15 per mag.
On the plus side Virgil Tripp, the original designer of the STI series of guns, is an excellent 1911 smith who could do your work and then finish the gun for you for a reasonable price and a good turn around time.
I have spoken with him on two occassions and he seems to be a great guy. One of the guys I shot IDPA with has a custom 1911 done by him and it is pretty damn close to perfect.
www.trippresearch.com