Before buying anything you should first make sure your machine will accept the upgrade you have in mind.
Minimum specs for Snow Leopard. Mac OS X v10.6 Snow Leopard -Read the Technical Specifications
You can now buy Snow Leopard from the Apple store on line again.
In the USA. Go to Mac OS X 10.6 Snow Leopard - Apple Store (U.S.)
In Europe or otherwise … call your National Apple Helpline. Find it at … http://support.apple.com/kb/HE57
In the UK … Mac OS X 10.6 Snow Leopard - Apple Store (UK) Where the last known price was £14.
Before you start upgrading, back up your files.
Once you have upgraded to Snow Leopard you will need to apply the Combo updater. Mac OS X 10.6.8 Update Combo v1.1 and follow with a software update to pick up any later improvements.