There is a sniper rifle on roof of abandoned gas station in detroit streets area. You can get on the roof by accessing ladder on the back side of billboard. Up there you will find a locked door with level 4 security console. If you open it you can navigate through corridors to the open window from wich you can jump on roof of gas station. Sniper rifle awaits in vent.
You can also stack some crates, barrels on top of container with dirt/trash, behind the gas station, and jump there that way or if you invested into augmentation that alow you to jump higher.