Buy a retail set of installer discs for it.
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
It's a pity you didn't keep a bootable clone of the drive. The $100 in a backup drive would have cost less than a replacement Leopard will.