I have already tried in arxiv but there are only 3 papers there. :uhh:
See if you can find his coolected works
in your library, or through inter-library loan.
Also inspires : http://inspirehep.net/search?p=exactauthor:R.Penrose.1+
Roger Penrose, 'Road To Reality'. ISBN 0-224-04447-8
Separate names with a comma.