Compared to other types of ammunition in the game, Sniper Rifle Ammo is rare to come by, but can be found when dropped by enemies and can be obtained by looting them.
As always, ammunition of all kinds can be found at any settlements' Gunsmith. Each ammunition box that is purchased contains 20 rounds of ammunition. Be careful to buy boxes one by one until listed as "not available" because it is possible to over-spend and buy more ammunition than the capacity of the weapon. Should a gunsmith not be present, check the general store in places such as MacFarlane's Ranch.
The following weapons use Sniper Rifle Ammo: