I'm busy reading the marvellous work of Victor Paul Furnish, Theology and Ethics in Paul, which includes a helpful orientation essay by none other than Richard B. Hays, a celebrated New Testament ethicist.
In describing the sources for Paul's moral thinking, Furnish notes three major sources:
1) First Testament. Furnish helpfully describes Paul's indebtedness to the conceptual framework of the Hebrew scriptures, but not in a legalistic fashion, as Paul does not use the First Testament as a rule book. Clearly these writings played a formative role in his thinking, but in the form of promises fulfilled and a revelation of the character of God, not as timeless principles.
2) Hellenistic Philosophy. Paul clearly employs the format if not the content of Hellenistic Philosophy in his ethical thinking by using virtue and vice lists, and other concepts like the Household code.
3) Furnish notes that the Jesus traditions provided much guidance and help in Paul's ethical thinking, but again not as a rule book but as a narrative framework within which to deliberate. By various allusions and perhaps even citations, Paul establishes continuity with the traditions of Jesus.
However, amidst these sources, Furnish also notes the creative genius of Paul.
Without detracting from the importance of the insight that Paul’s concrete ethical teachings are in large measure derived from traditional sources, both Christian and non-Christian, it must also be emphasized that he has, in most cases, exercised his own critical judgement in selecting these materials and has assimilated them into significantly new contexts. He has not been simply a collector and curator of miscellaneous moral advice; the impress of his own interests, perspectives, and objectives has been left upon them to a greater extent than Dibelius, for example, was inclined to acknowledge.
We thus see Paul as a person formed and shaped by his traditions and culture, but a slave to neither in his thinking and practice.
 V. P. Furnish, Theology and Ethics in Paul. New Testament Library. (Louisville: Westminster John Knox, 2009), 81.