Only the Bloodtracker and Praetorian careers have access to the ability and only with one specific weapon. The only other way to get it is for your GM to give it to you or make it available.
True, and I would also strongly advise against providing homebrew sources.
Weapon Master is the main advantage of Mighty, and what makes it stand its way somehow to Skilled and Gifted characters.
Skilled and gifted characters gaining it on the top of their abilities (Signs&Portents and extra attack for Skilled, and ability to boost and cast spells for Gifted) would make them heavily OP and make Mighty archetype even less useful than it is now.
If you really want characters that can add two dices to damage on non-charge attacks, than I suggest you to just make an Epic powered campaign and let everyone take 2 archetypes. That way every char becomes OP, but at least they all do it in the same way and you can build the whole campaign knowing that you are dealing with characters with the epic-ness level of Haley or the Butcher.