I believe you get a single random nirncrux item from the end of a questline in Craglorn.
Outside of that, all crafted nirnhoned items are researched and crafted. Nirncrux the trait material is the reason they are expensive. It is found rarely by harvesting nodes in craglorn. Armor pieces are less expensive than weapons because fortified nirncrux is used to make armor only costs way less than potent nirncrux used to make weapons.
I believe , not sure , there is a small chance you can get Nirnhorned items from weekly trial reward . Other than that , I know for a fact that you can get Nirnhorned weapons from vMA and it's weekly rewards .
Where are you suppose to get nirnhoned weapons and armor? Only time I've seen them is in guild traders but they sell for so much.
Funny I gave you the answer on a different thread to this question.
When you complete the craglorned main quest you get 1 nirnhoned item to research. You can also get them from VMA on Maelstrum weapons and rarely as a reward from trials and top tier pvp end of campaign rewards.