The armor will appear in the game only after completing the first end game faction quest. It matters not what faction you join with, long as you start one of them. After which, NCR Veteran Rangers will begin to appear across the Mojave Wasteland.
They started appearing in mine before I even met the ambassador (I've sided with the NCR). I was around level 22-24, but I had done a fair few NCR missions already. When you start hearing the line "So I hear the Rangers from Baha are coming" or whatever, they start popping up at all the Ranger Camps, as well as Camp Golf.