Might I ask why you can't upgrade to OS X 10.5.8? Is there a conflict with some other software on your computer? Or are you needing a PowerPC version of the application?
Not sure which version you would need, but here's a download list of older versions that might help.
hello there, did you find a solution to the Skype download for your iBook? I'm having the same problem, the Skype icon vanished from my dock this morning and it's turning into a long day. I haven't been on this site for ages. If you have a solution can you email me on ...email@example.com. Cheers ...b