Tell me if it can be bought at some merchant. I believe it can only be found in the world.The best part is that the more expensive the weapon, the more expensive the repair. That's actively discouraging you from using or upgrading unique weapons.
Let me tell you about adra ban
That's my point. Didn't you see my rant about it? I think the mechanic of adra ban - a few other crafting items too but adra ban in particular - completely undercuts the otherwise extreme diverse itemization. I think it's one of those choices made in design because it "made sense" and no one really thought about the practical impact in game. In essence, it doesn't mean crafting is a slow progression (gold does this already), instead it means once you Legendary a weapon, that's it: you're using that weapon until endgame no matter whatever else cool shit you find.