Did you try the ARMORY????????? Seeing as how that's where the guns are generally stored & all.
Also...you don't have to look very far for a sniper rifle. All you have to do is head north out of Megaton past the school. There is a broken down overpass there. On top of that overpass is a group of Raiders. The leader of the group has a sniper rifle that's in pretty good shape.
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I think you can get a sniper rifle off Stockholm ( the guy above the entrance) if you kill him in such a way that the rifle or part of his body lands in a reachable area. As for the Fusion Pulse Charge, needed to rig the bomb to explode, you can get one of Mr.Burke in Moriarty's Saloon if you agree to help him(NOTE: this makes you lose massive Karma(1,000))
Exit Megaton, and loop around back down the WEST side of Megaton. Follow the wall of Megaton until (using your PipBoy, set to Local Map) you reach what would be 7 O'Clock on a non-digital clock (SSW). There should be 3 Mole Rats when you are close. With Megatons Wall on your left, and the wasteland rocks on your right, look for a rock w/ 2 or 3 skinny trees growing right next to it. Walk inbetween the trees to the rock and activate the rock like a door. Inside will be the sniper rifle and some other goodies. The rock isn't far from Megatons wall. It's only a few steps, so don't go out wandering.
Imagine the path you are walking as you follow Megatons Wall as a sidewalk. Megatons Wall is on your left, and on the right will be the rock. Not way far off the path, but right next to it. Don't forget, when you see 3 Mole Rats, you are close.