If you are in the US call Apple Customer Support 1-800-767-2775, provide the Serial Number and specifications of the Mac, and for a reasonable fee, they will supply a replacement set of system discs (if available).
The discs will be for the original version of the OS that was pre-installed when the Mac was manufactured.
If you are in another country:
http://support.apple.com/kb/HE57
If you are running Lion or Mountain Lion you won’t have been given disks. Instead you use the recovery partition or internet recovery:
http://support.apple.com/kb/HT4718