Welcome to Apple Support Communities
If your computer came with Leopard, call Apple to get replacement discs > http://support.apple.com/kb/HE57 They will ask you for your serial number. If you don't know it, open > About this Mac, and press twice Version, so the serial number will show up.
If your computer didn't come with Leopard, you have to search Leopard in Amazon or eBay. It's expensive, so you should consider the option to buy Snow Leopard and upgrade to it instead of having this old Mac OS X