Sometimes you find it randomly in the armourer/faction blacksmith shop. Or at the travelling merchant.

It starts spawning in faction trader after you reach 70 relation with faction and became part of the family (in dialog with robb stark, russey bolton, rickard karstark, tywin and probably someone else there is an option that sounds something like "i would like to join your family")

引用自 Dergardos：

It starts spawning in faction trader after you reach 70 relation with faction and became part of the family (in dialog with robb stark, russey bolton, rickard karstark, tywin and probably someone else there is an option that sounds something like "i would like to join your family")

thanks bro.