Q: Power Macintosh G5 with no OS. What/where should I buy?

Leopard is the max OS. Mac OS X 10.5 Leopard installation system requirements

http://support.apple.com/kb/TA24950

Leopard is no longer available at the Apple Store but may be available by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753).

If you can't obtain a retail install DVD from Apple, look on eBay or Google the installer part numbers to possibly find at an on-line store. Here's what to look for:

MB427Z/A  Leopard 10.5.1 install DVD

MB576Z/A  Leopard 10.5.4 install DVD

MB021Z/A  Leopard 10.5.6 install DVD (single user)

MB022Z/A  Leopard 10.5.6 install DVD (5-user family pack)

Installing Mac OS X 10.5 Leopard

http://support.apple.com/kb/HT1544

Mac OS X 10.5 Leopard Installation and Setup Guide

http://manuals.info.apple.com/en/leopard_install-setup.pdf

After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update

 Cheers, Tom

Leopard is no longer available from Apple. One source for unopened Apple software is http://www.hardcoremac.com .

If you go to the used market, make sure you get a black-faced retail disk and not a gray-faced disk. The gray-faced disks are locked to the system type they were delivered with, and Leopard wasn't delivered on PowerMac G5's. Also, there is a black-faced Leopard upgrade disk, which requires OS X 10.4 to be on the system, so you wouldn't want that one either.

It is a risk, but I suspect many of them have sold it, the buyer copies it, then wants to return it.