Hello Dave@Streets, this is a forum link for purchase info:
macjack post.
It is best to look and search in other catagories such as Mac OS X 10.4, Mac OS X 10.5 or PowerMac.
Select the +more options+ above Logout after choosing the catagory you want to search. I have a PowerPC CPU also so I too can only go to Mac OS X 10.5. From what I have gathered so far: Max out your RAM, Back-up to an external firewire HD, and check requirements for iTunes 10 and Mac OS X 10.5 for your system.
Good Luck,
A