There are a few places to get this throughout your playthrough, but if you want to get it out of the way then the first time you get the Sniper Rifle is perfect.
When you first approach Hall of Heroes Plaza, a sniper will fire at you causing Elisabeth to pass you a Sniper Rifle to take him out. Once he is dealt with, move forward and a bunch of soldiers will come out and attack you. Stay in cover as best you can and kill them with your rifle. Reload checkpoint, which will take you back to the front of the plaza, and rinse and repeat until you get to 30 kills.