Hunt for Protectron's. They use energy weapons, so they are all loaded up with it.
Are people seriously having problems finding ammo for weapons? The only one I don't usually find a lot of ammo for is the 44 magnum. All my other weapons (which, in order of usefulness are; combat shotgun, assault rifle, hunting rifle, and 10mm) have a steady 400 count in the ammo, as it's easy to come by. So are caps, since basically everything is sellable. My favorite thing to do, with a high repair skill, is to grab up all the raider armor or whatever other armor of what you are killing, combine them all together through repair, and then turn around and sell them. You don't lose any money by combining them, and it cuts down on weight. Rivet City is a good place to buy ammo, so is Moira Brown - these places restock their shelves every few in-game days or so, so check back with them. Also if your lockpick is high enough, the armory in Megaton has some nice stuff.