Since you have no installation disks, you have two alternatives-
1. Purchase a retail Leopard installation disk
2. Call Apple and provide them with your Macs serial number- and tell them you need to order replacement "fulfillment disks" which is their term for the restore disks that originally shipped with your computer. There will be a moderate charge (when I recently ordered some for a DP 2.7 G5 that I am going to sell and the charge was $32 shipped for the two disks). The disks will be identical to the original disks, so if your computer shipped with Tiger the fulfillment disks will be Tiger too. If it shipped with Leopard, then the disks will have Leopard, etc.