when you log in and start in falconreach, you must go right. Keep going till you come across and black cloaked and hooded figure. Ask him about the doom weapons shop and you can buy it from him unfortunately for 25 grand! 25,000 if you're stupid. There is nowhere else that I know of where you can earn them. Those doom weapons are really strong and to make them stronger, go to artix and convert them to light. Then it becomes super strong.
Hope this helped.......