The best place to obtain one would be eBay. Just make sure you get the correct drive for your model.
Depending on which country you live, there are some parts companies in the US/UK that I know of which supply refurbished drives for PowerBooks. They wont be as cheap in some instances as eBay, but guaranteed to be work at least.
If you need instructions on fitting a drive to your PowerBook, try going to ifixit.com or web search ifixit powerbook G4
Best of luck
Before replaceing the drive, try a drive cleaning disk, the type with tiny brushes. A friend's MacBook Pro had the same issues with his Superdrive and an Apple tech pronounced the drive dead. I loaned him my cleaing disk and the drive now works fine. It was simply dirty.
Cleaning disks--US$5-15. New drives--much more.