To expand a bit on the previous post-
If you are going to get it on eBay, just insure that you are getting a full retail Tiger installer disk (which is black and has a giant X on it) and not some of the many System Restore (gray) disks that are offered there which are only good on the exact same model/revision of Mac they originally shipped with (and I just checked and no iMac G4 shipped with 10.4- they were all 10.3, so no System Restore disks will work with yours).
Additional sources for new and used retail Tiger disks are Amazon.com,
LA Computer (home of great deals on AppleCare),
Hardcore Mac (expensive but in stock) and
FastMac (upgrade disk for machines with 10.3 installed already as yours probably is- if so this is one of the cheapest places to get it).
Keep in mind that all these will be 10.4.0 installers, but the free updaters to the last version (10.4.11) are available at Apple's download page.
Good luck with your search.