Where I can buy an original OS X Leopard PPC disc for my iMac G4?
Besides maybe posting to Craigslist for a better deal, 10.5.6 is still available for sale on Amazon.

Please know there was no PPC-specific version of the installer (it works cross-architecture), so if you find one it will work on that Mac (as long as its processor is 867 Mhz or faster w/ 9 GB of free disk space and a DVD drive). It was only Tiger that had separate installers.