Dead slaves can be found in five static locations throughout the Capital Wasteland. They hold the slave map on them. Additionally, slaves appearing as part of random encounters may be found dead due to nearby hostiles and also have the slave map on their corpse.
Directly northeast of Big Town, on the side of the road.
In northern area of Metro Central. Walking from the Dupont Circle station entrance, once you reach the tracks take the first left through an opening between the tracks. The body is located to the right of the crashed train just through the opening.