Leopard's the highest you can go:
Leopard 10.5 Technical Requirements - http://support.apple.com/kb/HT3759
It isn't easy to find (certainly not from Apple), nor cheap.
Texas Mac Man, Leopard disc sources - https://discussions.apple.com/thread/3636662
"a brody" post about upgrading to Leopard - https://discussions.apple.com/thread/3373795
Call Apple Customer Support 1-800-767-2775 (telephone numbers for other countries - http://support.apple.com/kb/HE57). Provide the serial number and specifications of the Mac. They may be able to provide a replacement set of discs for a fee. These discs will be for whatever OS was installed on the computer originally, so if you have an upgraded version you will need to obtain the discs some other way.
I believe that those disks are upgrade disks and require an OS present to install.
To be precise, any retail OS X, 10.5x Leopard version can be used in a G5.
A G5 can use OS X up to 10.5.8.
For that matter, most all retail versions of OS X, 10.4x Tiger can be used in a G5. A possible exception would be a pre 10.4.2 OS X retail version in a dual core, Late 2005 G5, which would require 10.4.3 or later.
In 2003 and 2004 G5s, retail OS X, 10.3.9 Panther can be used.
Each machine has a minimum OS version which is dictated by production date.
Find out exactly what your machine is by entering the serial number here:
Find the minimum OS by model:
OS X, system specific gray discs are tailored to a machine at production. Purchase of these types of discs are completely at your own risk and shouldn't be purchased without in depth research.