It can be found in the Museum of American History buildings office floor, inside a display case.
Detailed info from the Wiki.
* The door the to Lower Levels is located to the left of the door to Underworld in the great chamber of the Museum of American History. Fawkes can be found waiting here if the player has progressed far enough in the main quest and has not recruited him yet. * The player should head up the stairs to the upper level upon entering the Lower Levels. The ground floor consists only of a cafeteria. The area is inhabited by Feral Ghouls and Feral Ghoul Roamers, even Glowing Ones depending on the player's level and difficulty level. It is worth noting that there is a turret located in a room adjacent to the top of the stairs on the right that will fire upon the player almost immediately. It can be controlled using a nearby terminal. The door to the Museum of History Offices is against the back wall of the upper level, past all the shattered display cases. * The Museum of History Offices are also prowled by Feral Ghouls. A player can expect heavy close quarters combat and room-to-room fighting in the area. Various memorabilia relating to Abraham Lincoln can be found in this area; see the article for their exact locations. Through a lightly-locked door of steel bars, there will be an open chamber with a high ceiling: the remains of a library or archives. A ceiling-mounted turret is located in the center of this room, usually occupied by firing on the Feral Ghouls around it. The player can opt to let the turret finish its work before destroying it, but it usually takes quite a while to fell its targets (and you won't get the experience). Either way, the staircase in this chamber leads to an upper floor of similar layout to the area below. The Feral Ghouls up here may be wounded or dead due to the turret's attacks, which makes this floor easier to secure. The office in the center of this floor contains the display case that houses Lincon's Repeater. It is surrounded by Feral Ghouls, so one can expect a fight. The chance to encounter a Glowing One is high, but if the player has already encountered a few on the Lower Levels, there usually won't be another here. Lincoln's Repeater is in the display case at the end of the central room farthest away from the staircase.