What is the OS version installed on it?
If it is 10.7 or 10.8 - About OS X Recovery - http://support.apple.com/kb/HT4718.
If it is 10.5 or 10.6, you can call Apple, and they can ship you out replacement disks. I think you need 2, and they are about $16.00 each. To contact Apple - http://support.apple.com/kb/HE57.