Call USA Apple Customer Support 1-800-767-2775 (telephone numbers for other countries - https://support.apple.com/HT201232). Provide the serial number and specifications of the Mac. They may be able to provide a replacement set of discs for a fee. These discs will be for whatever OS was installed on the computer originally, so if you have an upgraded version you will need to obtain the discs some other way.
If you need a retail set of discs it is extremely unlikely Apple will have a set. If you do try Apple you may have more luck calling support than sales since sales will just tend to look in their catalog and Apple stopped stocking Leopard discs many years ago. More likey is you will have to try third party sources:
Texas Mac Man, Leopard disc sources - https://discussions.apple.com/thread/4466537?answerId=20151842022#20151842022