You do not have to farm this, it will come naturally while playing. When you’ll actually get this depends on the level of your world difficulty. On world tier difficulty 6 and 7 (was stuck at 5 for a long time), I got this at the end of Trench Town around character level 17 due to the
If you’re on a lower difficulty or die less than me you’ll likely get this sometime in The Quarry or a bit later.
You can make it easier by picking your favorite gun and using Iron or Titanium to keep it leveled up along with you (much better than equipping whatever drops).