I'm looking for an easy way to get my hands on high-ranking Legion armor ala Centurian or Praetorian without having to kill anyone (or, if I have to, not getting in trouble for it).
I can't remember, isn't the head guy at Cottonwood wearing a set? You could either draw NCR patrols over from Searchlight using low powered weapons to tap them into chasing you, or play toxic waste management and clear out the entire village (no guarantee of no rep hit there, I have not had the opportunity to do that myself).
Sneak behind your victim, put any explosive (dynamite, mine, grenade) in his pocket, if you pass the pickpocket check he goes Booom. You will get exp, the armor and no negative rep.