As an active companion, Robert Joseph MacCready will frequently provide fusion cores in lieu of ammunition if he is equipped with a set of Power armor.
• Arturo Rodriguez in Diamond City usually stocks a few.
• One can be found in the Museum of Freedom's lowest floor behind a security door (Novice terminal). Sturges asks the player character to retrieve it during When Freedom Calls.
•Another is in ArcJet Systems in the room where the player character can restore power to the elevators.
• One can also be found in the mole rat den under the Red Rocket truck stop.
• Up to five can be found in Recon Bunker Theta during The Lost Patrol.
• One can be found in the office of Cambridge Polymer Labs, either by picking the master lock or successfully finishing the laboratory's research.
• One is located in Mystic Pines (Northeast of Lexington) in the basement. The door to the basement is locked (Expert) and requires 2 point in Locksmith perk to pick.
• One can be found at the Starlight Drive In in the building behind the screen, with a locked (Novice) door.
• In College Square, inside a trash bin behind the building beside an electric power black box.
• Up to five can be found in The Castle's armory (including from Sarge sentry).
• Three can be found at Greenetech Genetics on the top floor, two behind a security door locked at master level, and one behind a door further on locked at advanced level.
• One can be found on the main road north of Poseidon Reservoir at a military checkpoint, in a power armor.
• One can be found east of Finch Farm, in the pump station across the river. Pass code is written on the wall, below the door.
• Erin Combes will give one for finding Ashes in the quest Here Kitty, Kitty.
• Two can be stolen off suits in the The Prydwen's maintenance bay by sneaking behind them (Paladin Danse will disapprove).
• Two can be found at the lake gift shop in a suitcase during the mission where you fight a group of raiders to liberate a village.
• One can be found inside a waystation when following Dogmeat during the Reunions quest.
• One can be found in the church of the Quincy ruins.
• One can be found inside the Wreck of the FMS Northern Star.
• One can be found south of the elevator on the bottom level at Malden center
• One can be found behind a mag-locked door (unlocked via terminal right above it) in the Mass Fusion containment shed. The area is heavily irradiated.
• One can be found in The Railroad's HQ escape tunnel near the exit behind a security gate.
• One can be found in the Railroad's old HQ under Slocum's Joe. The fusion core is best accessible from the escape tunnel.
• There are three fusion cores inside Four Leaf fishpacking plant, two inside (one of them behind a mag-lock door) and one on the roof of the building.
• At least one can be bought from Proctor Teagan on the Prydwen
• One can be found in chest near the robot in the center of the Robotics Disposal Ground
• Four can be found at Hardware Town.
• One can be found in the eastern hut at Weston water treatment plant.
• Can be bought from Daisy in Goodneighbor.
• One can be found in the Concord Civic Access, in a dark corner opposite the small drug cache that the mirelurk guards.
•One can be found on the southern side of the Wilson Atomatoys factory.
•Three can be found at the Gunners plaza, two behind a novice locked door in a room on the opposite side of the elevator on the roof. One other one can be found on the center of the roof behind a door with a generator, running the fusion core.
•One can be found in the Longneck Lukowski's Cannery in the basement area after the sewer section; be careful of mines.
•Two can be found on a shelf inside the train cart strongroom in Goodneighbor during the quest The Big Dig.
•One can be found within Malden Drainage, an unmarked location at the end of the canal just north of the Taffington boathouse.
•Four can be picked up in a parking garage west of Milton General hospital. They are a part of one possible reward for completing the trap-ridden maze inside the parking garage.
•One can be found inside Boston mayoral shelter, in a room off the basketball court with a sign above it that reads "Restricted Area".
• One can be found in the basement of Kendall Hospital.
• One can be found atop the roof of Atlantic Offices.
• The wandering weapon trader Cricket often sells them.
• One can be found in a power relay under the elevated freeway just west of the South Boston military checkpoint.
• One can be obtained for returning an Overdue book at the arcade building near the Revere Beach station for fifty tokens.
• One is on the floor under a shelving unit at the top of the left set of stairs of Back Alley Bowling in the General Atomics Galleria

All sentry bots encountered in the game carry 2 or more fully charged fusion cores.