Looking for OS X 10.5.8
Contact Apple Care on the web at http://www.apple.com/support/contact/ or by phone and they will ship you replacement DVD for 10.5 if you have lost yours. You can also find them used on Ebay and such. Lastly, local vendors, a friend or Mac User group are all viable sources of a loaner DVD.

If you use Apple, don't pay for support and ask for an exemption in case the product is vintage or obsolete. Let them know you want to order replacement media. You will likely have to pay a fee for the disc, but not for talking to the support people to find out the price and availability of the replacement OS. Also, have the serial number of the Mac handy to ensure you get the correct original media.


http://support.apple.com/kb/HT1159