This is a difficult problem, because I don’t know of any, except perhaps Plato. My own opinion is that all philosophers are sophists, including yours truly. The Western tradition, despite Plato’s warning is basically Sophistry, and we do not see much self-realization in this lot. They were trying to describe everyday mundane experience, and particularly the role of science. They were not going beyond that into self-realization or any type of spirituality for the most part. The closest thing we have to a purely nondual spiritual master is Meister Eckhart. Mostly those who advocated nondual perspectives were killed off by the Inquisition if they arose. Meister Eckhart was careful to say that if you had a vision, or some other conceptual or experiential psychic phenomenon that you have not really begun the way to god that must go though the emptiness of God’s essence, in order to arrive at self-realization. If you want self-realization then you best bet is Buddhism, Taoism or Sufism, i.e. some non-Western nondual tradition. But Ironically this does not mean that the Western worldview does not have a nondual kernel. The core of the worldview generates nihilism, but due to the fact that the core is fragmented showing signs of discontinuities, like the lines in the divided line of Plato, means that ultimately in the kernel there is nonduality there. That is why it could spawn Buddhism and Islam as nondual heresies. And ultimately to these nondual heresies that are rejected by the Western worldview must return home, i.e. realize themselves in their source and origin which is in the kernel of the worldview.
Thus in a sense, in spite of the fact that all the Western philosophers are Sophists, whose only goal is to portray the essence of mundane consciousness or being-in-the-lifeworld, in fact they all together are pointing toward nonduality of the kernel of the worldview. And this was made possible because if read in terms of nondual understanding, Plato set the stage for this possibility. This is because in Plato’s dialogues, we really have a hard time to distinguish Socrates from the Sophists, yet all the various characters in the dialogues together point toward the nondual kernel of the worldview as the inherited wisdom of Egypt. So just as the distinction between the wise man and the fools cannot ultimately be made, all of the characters (who were actual people) in the Dialogues together point toward wisdom via irony.
The Western dualistic tradition was very effective at stomping out nondual heresies. So much so that it is hard to name anyone who made a fundamental indication of nonduality in the tradition. Now most heresies in the West were extreme nihilistic reactions to the nihilism of the worldview like Gnosticism for instance. An excellent exposition of this is in Morris Berman’s Coming to Our Senses.