From what I can tell, it is not an orderable part for "do it yourself" install from Apple. Do you not have a drive in there now? If you have to ask how easy it is, you probably don't want to do it.
According to Apple, Tiger will run on a G3 PowerBook, but a G3 iBook is not on the list:
http://www.apple.com/macosx/upgrade/requirements.html
I got an ibook G3, 600 mhz, 256 of ram, cdrom, mac os x.1. I want to install a combo drive, can you guys tell me if it is hard to do? I want to do it myself, also can you tell me if the Tiger of my G4 desktop would install on my G3 ibook? I want to upgrade it the most that I can.

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