Best wa y to obtain one is cut off the head of a person who owns one aftet branding them a traitor. Melt that bad ass great sword down make 2 blades give one to your son lefty and keep the other for yourself.Stealing something so rare and obvious is a cause for war Id imagine. Not to mention they aren't lying around waiting to be stolen.
Cause all the riches in the world are ubable to buy one it either inhertied or taken like above. As even the great tywin never owned one nor his family until Ice became his.