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The only place I can think of off the top of my head is the super duper mart. In it is a laser pistol sitting on a counter with a few cells lying around that was the first place I encountered an energy weapon anyway.
When I made an energy weapons user, I was lucky enough to exit the vault near a few Outcasts. Two had laser rifles and one had a gatling laser. If you're lucky you might run into them. They'll be up the road that runs past the vault. If you do find them, you'll probably have to run down to Springvale and grab the frag grenades from the mailbox before you can beat them.
Finding ammo may be a slight problem, but it shouldn't be too hard if you're any good at gathering caps and use the merchants.