The 10.5 disks are only available to purchase from eBay or Amazon. However, depending on your machine, you can upgrade to 10.6 Snow Leopard if your specs are suitable.
The principal requirement is is that your computer is Intel based.
To check your system ...
Minimum specs for Snow Leopard. Mac OS X v10.6 Snow Leopard -Read the Technical Specifications
Once you have upgraded to Snow Leopard you will need to apply the Combo updater. Mac OS X 10.6.8 Update Combo v1.1 and follow with a software update to pick up any later updates.
Let us know how you got on.