You can install Leopard (not Snow Leopard), look on Craig's list or eBay, but make sure you get the RETAIL version, which is black and has a big X on it, don't buy the grey install versions, they won't work. You can try the Apple tore, but I am not sure if they still sell it. You will have to pay a premium for it, but I can;t give you exact figures, Google it.
Clean up your apps, get rid of anything you don't use before you upgrade, and make sure you have all your serial numbers etc for the new install. You can use Migration Assistant which sould bring just bout everything over.
Good Luck,
