You have to option open to you.
First you can buy a retail copy of Mac OS X 10.4. There are some vendors which sale a copy on auction sites.
You could upgrade your iBook with double the memory and provided it's above 800Mhz you could install Mac OS X 10.5 leopard. Which you will find the DVD disk more accessible then Mac OS X 10.4
The next option is to download and install a copy of Ubuntu which is compatible with your iBook.
If you wish to install Ubuntu then I would refer you to the following site for further information and support
If you're installing Linux for PPC, please understand the application limitations, as not all Linux based applications will work with the PPC version.
Best of luck