3 Spawns for Splicer Key Enemies
There are a lot of places where you can farm splicer keys. The easiest spots are near the patrol point of Plaguelands.
Every time you hunt down a Brood Mother you get a Splicer Key.
Best case scenario is that you have a fireteam and you stay at all 3 places. These special enemies usually spawn every 5 minutes or so and you can catch all of them and every fireteam member gets the splicer key from the other player.