Ubuntu for PowerPC is now a community supported platform (the announcement). This means that ISO downloads have mean moved to the ports section of the Ubuntu sever, and no longer on most mirrors. You can download them from
ubuntu-12.04-desktop-powerpc.iso
ubuntu-12.04-alternate-powerpc.iso
There is a more complete and up to date list of download locations at: Ubuntu PowerPC Downloads
Answers to other PowerPC questions can be found in the Ubuntu PowerPC FAQ. And there is useful information at Ubuntu PowerPC Known Issues