Fallout Wikia lists the following locations for the sniper rifle:
- 188 trading post - can be bought from the arms merchant after level 11.
Atomic Wrangler casino - one can be found on the top floor, first door of the Atomic
- Wrangler inside a very hard locked case.
- Caesar's Legion safehouse - one can be found in poor condition.
- Camp McCarran - can be bought from Daniel Contreras.
- Camp McCarran - on top of a bed in the First Recon tent, where Sterling is.
- Gomorrah - sometimes 3 in poor condition can be found in the room with gun shipments. Be sure you take them before you destroy the shipments with thermite during the quest How Little We Know.
- Gun Runners - can be bought from the Vendortron after level 11.
- Inside a (locked- hard) room in a ruined building, to the northeast of Mole rat ranch. Both of the doors are booby trapped into this building.
- One can occasionally be picked off a legionary assassin.
- Vault 34 - Two can be found in the Armory (Accessed by using the terminal in the Overseer's Office). They are in very poor condition.
- Two can be found on mercenaries outside of Jacobstown.
Hoover Dam - A few can be found on NCR troops at early levels. They are replaced by riot shotguns and marksman carbines later in the game.
There's also the Gobi Campaign scout rifle, a unique variant of the sniper rifle:
- Sniper's nest - Found in the Very Hard-locked gun case that is next to an ammunition crate with .308 rounds. There is no key to open it if the lock breaks.
You may also wish to look into the sniper rifle's bigger brother, the anti-materiel rifle, or the unique Christine's CoS silencer rifle if you have Old World Blues. The Ratslayer, found in Broc Flower Cave, also deserves a honorary mention for being an easily accessible, scoped gun that silently shoots 5.56mm rounds.