Hi there. Arsenic Bite is a random world drop item. It can drop from practically any activity, purple engrams, or chests. It's part of a large pool of weapons, so it's a small chance to drop and there's no way to specifically farm it. Best thing to do is just play the game a lot. Open every patrol chest you see, open public event chests, kill every combatant you see, grind an easy lost sector, open vendor rank up engrams, etc. Banshee also sells a number of world weapons that rotates weekly, which can include Aresnic Bite at some point, though he sells only fixed rolls.