The answer to your question is most probably that Bernard is referring to a passage in the Republic that he does not know directly but has seen quoted in one of the many authors listed in the previous answer. If I were trying to find the exact source, I would refer to the work of Stephen Gersh and Peter Dronke. Here are two starting points.

http://www.worldcat.org/oclc/15792144

http://www.worldcat.org/oclc/51009016

Via Porphyry (lost commentary on the republic) and Augustine?

Thordwald,

I think you can read these two links.My answer is not a definitive solution, but a guide to find it:

1.  https://www.ucd.ie/t4cms/Platonism,%20Medieval.pdf 

Specially pages 432-+434

2 The sources of medieval Platonism Until the fifteenth century.

"The only Platomc texts available in the Latin west were part of the Timaeus (17a-53b) and, from the mid-twelfth century, the Meno and Phaedo. Medieval Platonism was largely indirect, filtered through the writings of the Christian Fathers, especially Augustine. Gregory of Nyssa's Platonism innuenced Eriugena. Aspects of Platonism (for example, the theory of ideas) were also transmitted through Latin writers including CICERO. SENECA, Martianus Capella. Aulus Gellius. Macrobius, BOETHIUS, Cassiodorus and Isidore".

Sentences in bold characters are mine.

2. http://www3.nd.edu/Departments/Maritain/etext/hop33.htm

Captured view of Jacques Maritain Center : History of Philosophy / by William Turner, University of Notre Dame, department.

Lía Viguria