Welcome to the Apple Support Communities
Call Apple to buy Snow Leopard > http://support.apple.com/kb/HE57 Then, make a backup, insert the DVD and upgrade. Finally, open > Software Update and install the latest version
Snow Leopard = OS X 10.6.x
It is Not available as a download... It is a Paid Upgrade.
Do this first...
Check that your Mac meets the System Requirements for Snow Leopard...
Snow Leopard Tech Specs
If so Contact Apple in your Country to Purchase a Snow Leopard Disc...
In the US...
1-800-MY-APPLE or 1-800-676-2775
After the Successful Install, run Software Update to get the latest updates for Snow Leopard.
Be sure to make a Backup of your Current System Before Upgrading...