10.6 Snow Leopard came out in July of 2009 and was made obsolete in July of 2011 when 10.7 Lion was introduced. Unlike earlier Operating Systems which were removed from the Apple Store when a new OS came out Apple kept Snow Leopard around for a year until 10.8 Mountain Lion came out.
It might still be available from Apple. You will have to call Apple Customer Care 1-800-692-7753 or 1-800-676-2775 to purchase it. It may still be in the Legacy Products list.
If they no longer have any in stock you will have to buy it from eBay or Apple resellers that still have stock. But you will have to pay a premium since the DVDs are no longer being made. Snow Leopard DVDs are already up to $100 on Amazon.