I got one or two a long time ago. I remember getting one on Venus, but I'm not sure about the others I have killed tons of enemies on all the planets when farming exotic material or completing bounties, but I haven't had any drop recently. With that being said, maybe they only drop randomly when completing patrol missions on a planet. This is about the only thing that I do differently, Maybe, in order to get the random drop, you have to complete a certain number of patrol missions, so as to activate the random drop. Who knows, maybe it won't drop if you don't do patrol missions?

Aptrol on every planet just keep killing shit and spawning more enemies. It shouldnt take too too long if rngesus likes you.

I got 8 in a row back when i was ranking to lvl 40. Havent gotten one since, in fact id ont think i got one for a year or so.

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.