J. R. R. Tolkien introduced us to mithril.
It is an extremely rare metal, at least in Middle Earth, appearing as silver but stronger than steel.
It will most likely be rare in any world/milieu, but that doesn’t mean it won’t be encountered.
The following listing will help determine what kind of item might be found.
Exactly where it is found is up to you.
Dungeons and Dragons Colour 