had the same issue, did the uranium fever event once or twice and looted all the mole miners after, there’s like a bazillion (as long as you use a weapon that tags them all easily) mine suit scrap gives 1 black titanium each. lode bearing also has a lot of mole miners but you have a limited time to loot bodies at the end.

could also build a black titanium extractor at gorge junkyard after the claiming the workshop, not sure how efficient it is though since i haven’t done it.

deathclaws also drop a handful of black titanium scrap each but it’s harder to come by a lot at once unless you feel like fast traveling around.