You can get Power Armor from some merchants, from some dead unnamed Brotherhood members, and from Sarah Lyons at the beginning of the last main quest, 'Take It Back!'. The armor will almost always be in bad condition.
You can obtain Brotherhood Power Armor from dead unnamed Brotherhood members. You can obtain Brotherhood Power Armor, Power Armor, Enclave Power Armor, Tesla Power Armor, Enclave Hellfire Armor, Outcast Power Armor, Lyons' Pride Power Armor, Linden's Outcast Power Armor, Prototype Medic Power Armor, Ashur's Power Armor, Army Power Armor, Tribal Power Armor, and T 51b Power Armor. You can obtain Power Armor in many forms.