There's one way a few M9's might get into the civilian market.
In all the sudden interest in the government giving the local police departments arms, I was surprised to find they're giving out 1911 .45 automatics.
Also listed were 9mm pistols, which almost certainly means the M9.
Over the years, some departments have sold some of the guns given to them, including some M14 rifles showing up as sold to civilians.
It's possible some departments might sell some M9's they no longer want.
Apparently the government hasn't been too interested in equipment they gave to the police being disposed to civilians.