Who sells yak armor in freminnik?

nobody. the only way to obtain it is to kill yaks on neitzinot for their hide, then bring them to the tanner on neitzinot, he is the only person who can tan tak-hide, then, assuming you have the reqired crafting level, use a needle on the tanned yak hide with some thread in your inventory at the time and you will be able to make it into armor.