If you are using Snow Leopard, OS 10.6.8, you can download Lion (or any OS beyond 10.7 up to and including Yosemite, OS 10.10.5 (assuming your machine is "qualified for Yosemite)) for free.
To further honestone's comment, OS X full versions are only available from the Apple App Store. OS X Lion (10.7) and OS X Mountain Lion (10.8) are available for $19.99 each from the online Apple Store. You will then receive a redemption code via email to use in the Apple App Store to download and install. I don't believe that OS X Mavericks (10.9) is available at all any longer. OS X Yosemite (10.10) is a free download, available via the Apple App Store directly. They are all available to re-download if you have previously downloaded them in the Purchases section of the Apple App Store when logged in with your Apple ID.
Of course, compatibility depends on your exact Mac model and currently installed version of OS X. OS X Snow Leopard (10.6) is required to use the Apple App Store.
Surprised that OS 10.9, Mavericks, is no longer available at the Apple App Store. Also, I believe that when you obtain OS 10.7, Lion, OS 10.8, Mountain Lion, and OS 10.10, Yosemite, you'll obtain the last/latest versions of each.
Surprised that OS 10.9, Mavericks, is no longer available at the Apple App Store. Also, I believe that when you obtain OS 10.7, Lion, OS 10.8, Mountain Lion, and OS 10.10, Yosemite, you'll obtain the last/latest versions of each.
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