NCR merchant at the Dam has the largest quantities of ammo. After that? Probably the Great Khan armorer or the Gun Runners.
Yup. Hoover Dam Quartermaster, Great Khan Armorer, the Gun Runner Shack, and Alexander under the 188 overpass. I use Vance's 9mm SMG all the time. Great weapon, especially against Cazadors. Each of those guys usually carries lots of 9mm.