It's not clear to me if you're looking for a New Hard Drive, or a 10.5 Install Disc.
If 10.5 you need a Retail Install Disc...
* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
minimum system requirements
* 512MB of memory (I say 1.5GB for PPC at least, 2-3GB minimum for IntelMacs)
* DVD drive for installation
* 9GB of available disk space (I say 30GB at least)
Classic/OS9 Apps no longer supported.
May be available from Apple again???
Trouble is Apple no longer sells it, check eBay & such for the Retail version, not the Gray Discs...