The oldest source for Alcibiades is Thucydides, but you'll have to picking through the whole history. Alcibiades only played a small part in even the Sicilian campaign. Xenophon also picked up his story in the Hellenica. Plato had him as a character in the Symposim. Your best bet is probably Plutarch's biography, here. Plutarch is admittedly not a historian. He's writing character sketches, and he's willing to pass along apocryphal stories if he thinks they illustrate the subject.