I didn't see any point in the game to get this naturally so after I had completed the game I selected chapter 25 via chapter select and then gave myself a sniper rifle via the reward options in the pause menu.
The first time you encounter the guardians the game forces you to use the AK 47 but after that there are two more opportunities to get it. The easiest is when you get to the massive battle between soldiers and guardians in the temple. Make sure you have the game on easy and you can even enable one hit kills and infinite ammo using the tweak menu.