I did quests in zone one until the Karwla appeared.  I found the new Armoury and the Black Market, and searched almost all of the map.  I never came across a focus replacement mutator,   Other than that, the mutator system is the best it has ever been in my opinion.  (I also played the Alpha or very early closed Beta version.)

Thanks,

the focus mutator will be found in special quests, comming in next build.

Or maybe we add them as common loot from fallen enemies, just like the normal (trait) mutators.