Vault 34 - Two can be found in the Armory (Accessed by using the terminal in the Overseer's Office).

One can occasionally be picked off a legionary assassin.

Both of the doors are booby trapped into this building.

Inside a (locked- hard) room in a ruined building, to the northeast of Mole rat ranch.

Gun Runners - can be bought from the Vendortron after level 11.

Be sure you take them before you destroy the shipments with thermite during the quest How Little We Know.

Gomorrah - sometimes 3 in poor condition can be found in the room with gun shipments.

Camp McCarran - on top of a bed in the First Recon tent, where Sterling is.

Camp McCarran - can be bought from Daniel Contreras.

Caesar's Legion safehouse - one can be found in poor condition.

Wrangler inside a very hard locked case.

188 trading post - can be bought from the arms merchant after level 11.Ītomic Wrangler casino - one can be found on the top floor, first door of the Atomic.

Fallout Wikia lists the following locations for the sniper rifle:

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