Well the first, some scopes are for specific weapons, for example, The Pso1(or whatever can't remember the name) scope is for the AKM and etc. The SUSAT scope is more advanced weapons. Second-Well, the high ranked Stalker's(Loners) often have the Vintar Sniper Rifle, wich is my favourite.
You can find a good sniper rifle while performing the "Reach the military base" mission - comes right after Yentar and talking to the old man in Red Forest. There's a sniper up in a tower. You know the drill - hack'n'slash - get his weapon