Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
With Font Book open go... File>Show Font File (Apple-R)

That'll show you where Font Book has stashed them in a new finder window. You can then copy them to wherever you want...

If it's a Postscript font, make sure you get both files (Font Suitcase & outlines)
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.