There's only 4 so once you get them you will not see any more - have you seen any so far? Basically you just keep playing on each of the 4 main planets (Moon, Earth, Venus, Mars) and they drop randomly. I got all mine on story missions and patrols. Once you have the 4 and turn them all in to the gunsmith, you have to wait until the 7th of the month for the start of the Sleeper Simulant missions, which will simply just show up on Earth - called First Firewall. You have to do this mission on the 7th (based on PST time) or it will disappear again. Once you complete that first mission, you can continue the rest of the missions any time.