OS X Snow Leopard

What is the last Cubase version that supported Snow Leopard?
I have a completely functional working system and I don’t want to mess with upgrades, etc.

Cubase 6.5 was the last version that was supported for 10.6. http://www.steinberg.net/index.php?id=downloads_cubase_65&L=1

Thank you. Is there a possibility to purchase it somewhere? Somehow?

Cubase 6.5 is discontinued and is no longer available for purchase. A Cubase Pro 8 license could open Cubase 6.5, but you will need to install the archive version of the eLCC from http://www.elicenser.net/en/download_archives.html