Since 7.04 Ubuntu, PowerPC downloads of Ubuntu are not available on all Ubuntu mirrors. Below you'll find links to the ISOs on the main Ubuntu server, as well as, help on finding mirrors.
Please review the PowerPC FAQ and the PowerPC Known Issues pages before installing. All the information you need for a successful install should be in those pages, so make sure you check them out first before wasting your time and effort on needless internet searches. Please do tell somebody on the Apple forum if you think the instructions are unclear or are missing something, but it would be great if you could make the improvement yourself!
You may also wish to read the general release notes.
Long-term support releases
14.04 Trusty Tahr
Lubuntu (LTS, 3 years)
Desktop live install CD, alternate install CD
Server (LTS, 5 years)
Netboot/mini
12.04 Precise Pangolin
Ubuntu (LTS, 5 years)
Desktop live install CD, server install CD, alternate install CD, and preinstalled desktop image for various platforms
Kubuntu (LTS, 5 years)
Desktop live install CD, alternate install CD
Lubuntu
Desktop live install CD, alternate install CD
Xubuntu (LTS, 3 years)
Install via the mini/alternate CDs following the instructions in the PowerPC FAQ.
Netboot/mini
Netboot/mini
Other releases
Lubuntu
Desktop live install CD, alternate install CD
Kubuntu
Desktop live install CD
Server
Netboot/mini
Daily build of the current Ubuntu development release
The following iso's are available for testing. In order for an iso to be released it must complete a series of test cases - volunteers are needed for this! Please also report bugs so that they get fixed prior to release.
Ubuntu MATE
Lubuntu
Live CD - http://cdimage.ubuntu.com/lubuntu/daily-live/current/
Alternate CD - http://cdimage.ubuntu.com/lubuntu/daily/current/
Ubuntu-Server
Netboot/Mini
Old Releases
More specifically
Mirrors
This is not an complete list (please add to it). Using the list of official CD mirrors and a bit of ingenuity traversing the mirror directory structures (for example, click on the Parent Directory/ or ../ link) you can probably find more mirrors that contain PowerPC releases. Most PowerPC ISOs are in a ports directory, but for others (e.g. 12.04) you can find them in the releases directory. Daily development builds can be found with other architectures in the daily/ (Alternate CD) or daily-live/ (Desktop CD) directories.
Academic Computer Club, Umeå University, Sweden
Silicon Hill, Czech Republic
University of Waterloo Computer Science Club, Canada
LeaseWeb, United States
Technische Universität Dresden, Germany
Internode, Australia