For all later versions, you must CALL the Apple Online Store in your region and arrange to purchase the 10.6 "Full Retail" Install DVD. In most places, it is not "on the shelf" and requires you to talk to an agent.
Once you install from the DVD, you will have 10.6.3. You can then use Software Update to update to 10.6.8, which includes the Mac App Store. The Mac App Store will allow you to make additional Upgrades to later versions if you wish. These will all be by download (no physical DVDs are issued).
10.7 is available to all Mac Pro models, but it is also not "on the shelf". You will need to call to arrange a Download through the Mac App Store.
10.8 is only available to Mac Pro 2008 and later. It can be purchased directly once you have the Mac App Store software provided in 10.6.8
What happened to the $69 SL 10.6.8 + Lion flash memory card? Gone and no more?
When you do download, before installing, which will self destruct and remove the installer package, make backup copies. You can also find instructions how to create your own flash installer for future use and clean installs.