startup disk for imac g5

Call AppleCare. If they don't have it, then the 10.4.6 retail installer can at least give you the ability to install the lowest possible operating system:

https://discussions.apple.com/docs/DOC-2541

And the 10.4.11 combo PowerPC update will help you bring yourself back up to where you currently are.

Mind you, you should backup your data prior to installing anything. There are 10.4 compatible versions of Carbon Copy Cloner and Superduper that can make an exact replica of your Mac to a Firewire hard drive, and Firewire 400/800 cables that can connect current Firewire 800 drives, if you can't find a 400 drive.

http://www.macsales.com/ sells lots of good external hard drives for Macs.

Do not buy a hard drive with manufacturer backup software installed as that may not remain compatible through updates.

You can't get Classic installed without the restore discs.

Some Apple usergroups may have people with the correct restore discs:

http://www.apple.com/usergroups/

Please remove the serial number from your profile.