Sniper's Nest near Cottonwood Cove (after Camp Searchlight at the bottom right of the map). It's near the top of a mountain or whatever that lets you overlook the entire settlement. There's a Very Hard (100 Lockpick) locked chest with the Gobi Campaign Scout Rifle, which is a greatly improved standard Sniper Rifle.
Otherwise, once you get some levels, the Gun Runners start selling the .50 Anti-Materiel Rifle, and some NCR rangers are equipped with it as well.
If you want something a bit more early, the Ratslayer in Broc Flower Cave (also near the bottom right of the map, but closer to the road you take while following the main quest) isn't bad.
ok i had the gobi scout rifle and it was almost identical in stats to the normal sniper rifle, it had 2 less Dam but 9 more Dps, it also has 1 more bullet in the clip, but it can'e use a silencer so i went for the normal sniper rifle with silencer, however the wiki page says the gobi rifle has something like 60 Dam and 217 Dps, this certaintly isn't the case in my game and i have 100 gun skill, what is the Dam and Dps for you guys??