I'm guessing in MoR there should be no (actually working to significantly prolong life) philosopher's stone, as not using that more widely would be altogether too crazy.
It could just be extremely difficult, known only to a selfish few, with the knowledge heavily guarded (perhaps using the Interdict of Merlin).
In canon, Nicolas Flamel may have toyed with being a bad guy, but in MoR he would definitely be a bad guy … for the opposite reason.
Update: This post has also been superseded - new comments belong in the latest thread.
The second thread has now also exceeded 500 comments, so after 42 chapters of MoR it's time for a new thread.
From the first thread: