Leopard 10.5 Technical Requirements - http://support.apple.com/kb/HT3759
Not easy to find, nor cheap.
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
Do note this system version no longer receives support from Apple. While it will run the latest iTunes don't expect it to keep up with technologies for long.