You can purchase Snow Leopard through the Apple Store: Mac OS X 10.6 Snow Leopard - Apple Store (U.S.). To access the App Store you must have Snow Leopard 10.6.6 or later installed. Mountain Lion can be downloaded from the Mac App Store for $19.99.
You can upgrade to Mountain Lion from Lion or directly from Snow Leopard. Upgrade Paths to Snow Leopard, Lion, and/or Mountain Lion