As the NCR Rangers are a threat to slaver operations in Northern California, they would do anything to hamper their operations. The slaver Vortis from NCR Town looks to procure the Ranger's map from their headquarters in the downtown NCR area and the Chosen One can provide him with it, for adequate compensation - $500.
Go to the Ranger's base downtown. The party must have weaponry holstered to get through the gate. South-east and behind Sheriff's office, there is a building with a garage inside. In the room with a workbench, search the table in the left corner. Take the map and return to Vortis.