You can purchase it from a number of vendors. Ebay tends to be less expensive; however, you must make sure you buy a retail disk rather than what is often offered is a machine-specific gray disk your computer will not recognize.
MacBook, or an iBook G4? Or MacBook Pro, or Powerbook?
(there is no MacBook G4, so perhaps best to find out what will be the best and newest OS that will work on your Mac)
