Biblia Patristica (a massive index of biblical verses used by the Church Fathers) is often still under copyright for recent volumes. I have written this post to point toward public domain/out-of-copyright volumes or legal previews (Google Books/Internet Archive).
This is a series of books (by Michael W. Holmes and others) that serves a similar purpose. Many university libraries have digital access to these. biblia patristica pdf free
Fourth-century authors, including Eusebius of Caesarea and Cyril of Jerusalem. Biblia Patristica (a massive index of biblical verses