Depends on your luck really (not the game stat). In my first playthrough I bought both a hunting rifle and a scope mod from her. Made early sniping a lot easier. In my second playthrough she was plain useless.
The hunting rifle is my favourite sniper when fully modded, I spend days now with fast shot ( less accuracy ) and standing up + walking trying to get long range headshots with it.
The best place to get one really early ( but quite dangerous ) is northen star cavern, just east of the mojave outpost, it's either that or a .44 revolver in a duffle bag.
Or close to Boulder city by one of the railway boxcars is one, these are all free and unowned versions too.
The gunrunners have a lot of caps, Crimson caravan do too, BoS, and the merchants that move between the areas can just keep adding caps to their amount.