I dunno, not really a reasonable thing for an American company to make. You see the 20" + rifle barrels because they can burn the powder and get higher velocity over 16". You won't see the performance over 16" on a 9mm; so there's really no reason for a US company to make one.
You might have to get Herron Arms, or Dlask or someone to make one for you. It should be a fairly routine barrel to make (especially no gas port or gas block step) but it'll still probably be a fairly pricey barrel.