You don't have to go to App Store to buy 10.6.8 Snow Leopard. Follow the notes below.
First, check that your machine is suitable for Snow Leopard. It needs to be a Mac computer with an Intel processor.Double check by reading the specs.
To buy the DVD, if you are from the USA go to http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard.
In Europe or otherwise substitute your national code in place of /us above.
In the UK see … Mac OS X 10.6 Snow Leopard - Apple Store (UK)
Before you start upgrading, back up your files.
Once you have upgraded to Snow Leopard, if your upgrade took you to 10.6.3 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 the most recent security updates.
This will take you to Snow Leopard and put the App Store in your applications folder.