I’m not sure where I came across this, but for those who do not have access to Migne’s Patrologia Latina or Patrologia Graeca, it seems that many of the volumes can actually be located by using the Google ‘book search’. The instructions I printed from the Internet are as follows:
‘To locate volumes of the Patrologia Graeca on Google Books, use the search “cursus completus series” (without the quotes). To get volumes of the Patrologia Latina as well, leave out the term “series”. NB: It is important to sepcify “full view”, otherwise nothing much comes back….
‘I find that the online displayed copies often seem to be missing pages; but the PDF available for download for them all does not.’