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...
1) Make sure your computer meets Snow Leopard technical specifications - http://support.apple.com/kb/SP575 This mostly means making sure it is an Intel Mac (your MacBook Pro is) and has at least 1GB RAM (at least 2 GB is recommended). To check RAM continue to "More Info...". It should say you have at least 1GB of "memory".
2) Apple no longer carries 10.6 in its stores or online. Telephone Apple to purchase a Snow Leopard disc. In the USA: (800) 676-2775 (contact information for other countries: http://support.apple.com/kb/HE57)
Alternatively you can buy from online retailers but anticipate paying much more than the original price which is the price still charged by Apple
3) Backing up your computer before upgrading is always a good idea.
4) After installing from the DVD, update your Snow Leopard installation using Software Update or manually downloading and installing the Mac OS X 10.6.8 Update Combo v.1.1 - http://support.apple.com/kb/DL1399
Be aware Snow Leopard's Rosetta is an optional install. And Lion and Mountain Lion it doesn't exist at all. So depending on how far you wish to update, make sure all your applications are Universal before you go to Lion and above. Also read this tip: