Ysgramor's Tomb has a chance of housing an Ancient Nord helmet.
You can find a number of different Ancient Nord helmets and boots in the Forsaken Crypt. These same pieces can also be looted off of ascendant necromancers if you are above level 81. If you choose to marry Aela the Huntress, you can buy all pieces except for the helmet provided you trade her some stronger armor first. It may also be found on the ghost at Kjenstag Ruins who can be found there between 4 a.m. The full set can be found by looting dragon cultists in Forelhost during the quest Siege on the Dragon Cult.
