Sensor modules are a moderately common item throughout the Capital Wasteland. They can reliably be found in toolboxes, metal boxes, and sometimes in the inventory of "miscellaneous" vendors like Crazy Wolfgang, Knick Knack, Karl, and Seagrave Holmes.
10 are in the Warrington station area: four on a shelf next to a bed in the northeast area of Warrington tunnels, and four (which must be stolen) behind a bed on the bottom shelf in Roy Phillips' sleeping quarters. Two more are in a crate on a bookshelf in the Metro Access & Generator, in the room where Roy dumps the bodies of Tenpenny Tower residents if you side with him during that quest.
7 can be found in the RobCo facility. In the factory floor section: three are on desks northwest of the entrance, one is across from the workbench, and 1 is on a shelf near three protectron charging stations in the northwest. In the offices and cafeteria section: one is on a table in the northwest corner, and one is on a table in the room northeast of the mainframe.
6 can be found in the Robot repair center: two on a shelf next to a first aid kit, three nearby the shelves filled with metal boxes (one on the floor), and one next to a computer terminal upstairs.
5 can be found in the County sewer mainline. One is near the terminal used to access the sewer management main room, next to a Big Book of Science. Two more are in the room to the east of that one, on a shelf. Two are in a crate in the room where Gallo resides, next to the Nuka-cola stools.
Point Lookout (add-on) 2 are in the Lighthouse interior: one on the ground beneath the stairs, and one at the top of the stairs. One is also part of the loot in the Underground lab given as a reward for completing A Meeting of the Minds.
Mothership Zeta (add-on) 4 are found in Mothership Zeta: three in the Cryo lab beneath the cryo pods in the middle of the map (one on a shelf, two in a white box). One is in the Engineering core, on a crate in the northeast corner of the main hub.