You Can Pickpocket Fusion Cores
Pickpocketing can get you in trouble but it also can be valuable if you pick the right person to rob.
Once you get tired of swiping stuff from regular old NPC's, try your luck with NPC's wearing Power Armor. If you're adept at pickpocking, you can actually steal fusion cores from enemies wearing Power Armor. That's big for two reasons.
One, fusion cores are essential because they power your armor. And two, it immediately puts them at a disadvantage because again, power armor needs a fusion core to function properly.