This is another very simple one. Go to the Bouleuterion of Athens to meet with Perikles's contact. Speak with the informant about the ostracism, and then make your way to the Logismos building in the Agora. When you reach the building, use Ikaros to scout the area. Tag all of the guards in the vicinity and then jump the wall from the western side. From this position you will havbe a good view of two patrolling guards in the courtyard. You can take them out with ranged skills or stalk them from the grass to attempt a clean assassination.
Once they are dead you will find one more guard at the door from the courtyard. Assassinate him to clear the way into the building, where you will find the Ostraka's marked on the ground floor and the first floor. When you have both, take them back to the informant. A brief cut scene will then initiate, after which you have a brief opportunity to speak with Socrates.
Note: Before you complete the next quest, 'Perikles's Symposium', you should be sure to complete any unfinished side quests in Athens, as you will not get another chance to do them upon your return at the end of the next chapter.
Did You Know? Although Socrates is widely considered to be one of the most influential thinkers in history, he never actually wrote anything (at least hitherto known) during his lifetime. Aside from relatively minor (though important as forms of evidence) contributions from Aristotle and Aristophanes, most of what we know about Socrates comes to us from the writing of two of his students; Plato and Xenophon.