Speak to Jules or Father Tully and ask about the Salvatores. Ask where they get their "lightbringers" (aka laser pistols), and he'll say he doesn't know. Go see Eldridge in the West Side of town and ask if the Salvatores buy the pistols from him. He'll ask to have one received for study, and will pay handsomely for it.
Although the promise is a pistol from the Salvatores, any basic laser pistol will do. If Louis Salvatore is served loyally, one can eventually ask him for a laser pistol before the third mission. Of course, outright looting the Salvatores of their laser pistols works too.