What I got from the various books is, most of the ones in Militia service were destroyed fighting either the Usurper or subsequent invaders. Kerensky also took some of them with him on the Exodus, and though lord knows I would KILL for a Mackie IIC, most of those probably got destroyed in the Pentagon Civil War.
That said, there were apparently TONS of these things lying around, so I don't see why it would not be possible to find and repair one.