Self Mining Cores
They are a random drop. I tend to find they are mostly found in Rare and higher end loot crates in POI’s, mostly the black Ultra Rares…. so you’ll have to go into the bigger abandoned outposts such as Bunker, Assembly Yard, Reactor, Mine, Drone Base and similar sized.
They don’t drop in the smaller easier abandoned POI’s.
Same can be said for whatever faction buildings you might choose to raid, but I don’t really bother too much with faction buildings to quote which type they might drop in.
You can also buy them from NPCs, some of the Polaris Trade Stations in orbit, along with other facilities you can find NPCs willing to sell them, but they’re in the 30-35k range.
Its likely you can also find them on vendors for other factions you are on good terms with.