I can finally answer this after using 3 different save files to test. This armor starts showing up after you complete quests from the NCR ambassador in New Vegas, which he gives you after you visit Mr. House. I had a save with a lvl 20 character without doing these missions, never saw a ranger once. The moment I was done with the Ambassador quests and he sent me to talk to someone at the Hoover Dam, I went to check the Delta post just once and there they were. I can't say for sure that lvl doesn't have anything to do with it, but I can definitely say they don't show up unless you've done that series of quests for the ambassador.