If you have their armor do you know how to obtain it without losing karma (or killing them if possible)

In the mission where you save the president, one will get shot. You can grab the armor from his body.

If you have high NCR rep, Col. Hsu gives you the key to the Ranger Safehouse. If you have the latest patch, Ranger Patrol Armour, NCR Salvaged Power Armour, and Ranger Combat Armour will be on a shelf in the first room on your right. They are all marked as faction disguises and your high NCR rep will be masked by that armour.

Yep I have It. Do loads of work for the major in camp macarren and eventually he will give you a key for a store room in the wastes there is ranger combat, ranger patrol and recovered ncr power armour in this store room.