That model came originally with 10.5.7, a version of Leopard. If you don't have the discs that came with the computer and plan to sell or give away the computer, then you need the original discs because they contain all the pre-bundled software along with OS X.
You cannot work around this. You can install Snow Leopard instead, but you won't have any of the pre-bundled software. See below:
You can purchase Snow Leopard through the Apple Store:Mac OS X 10.6 Snow Leopard - Apple Store (U.S.). The price is $19.99 plus tax. You will be sent physical media by mail after placing your order.
You can contact Apple Support to see if you can purchase replacement discs for your model, but don't be surprised if they cannot provide them. In this case you will need to look for them on eBay or Amazon.
Apple Store Customer Service at 1-800-676-2775 or visit online Help for more information.
To contact product and tech support: Contacting Apple for support and service - this includes
international calling numbers..
What to do before selling or giving away your Mac