To obtain a set of T-45 Power Armors, complete the quest When Freedom Calls. As you assist Preston Garvey in Concord, you will enter a roof building containing a T-45 Power Armor set and a minigun. However, you will only have one fusion core and likely run out of power shortly thereafter. Consider parking your original Power Armor set at Red Rocket Truck Stop, which has a Power Armor crafting station.