The Late 2008 model 5,1 Aluminum Unibody through the Mid 2010 White Unibody model 7,1 can run 10.8 Mountain Lion.
The Snow Leopard 10.6 DVD should still be available at this time from Apple for $19.99. 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. The App Store which is required to download 10.8 Mountain Lion is part of the 10.6.6 update.
If you wait to long and 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 over $100 on Amazon.