Fortnite Chapter 3: Where to get the Boom Sniper

Lt. John Llama will sell you the Boom Sniper Rifle for 600 Gold Bars in Fortnite Chapter 3 Season 1. It doesn't spawn anyplace else, for now, so you won't find it lying around or in chests. You can only earn a Boom Sniper Rifle if you eliminate a player who already possesses one.

Lt. John Llama may be found at the Llama Homestead, which is west of Crackshot's Cabin and southwest of the Logjam Lumberyard (otherwise known as the Nutcracker House or Christmas House). Unless another player gets there first, this is also the place where you'll always discover a Supply Llama in every match.

The Boom Sniper Rifle is an Exotic variant of the Heavy Sniper Rifle that uses explosive clinger rounds as its ammunition. It also has a larger magazine capacity than the Heavy Sniper Rifle, allowing you to fire five bullets instead of only one before reloading.  

A Boom Sniper round's initial impact causes only 10 damage (25 for a headshot), but the explosion that occurs a second or two later causes 60 damage, bringing the total damage per shot to 70 for a body shot and 85 for a headshot.