Snow Leopard isn't available as a download from Apple.
Once you get to SL, you can go directly to Mountain Lion (v10.8), which is downloadable through the Mac App Store. One of (if not the) reason you need SL, is it is the first version to support the Mac App Store, which is needed in order to purchase Lion or Mountain Lion (10.7, 10.8, respectively).
Also, your iMac needs to meet the minimum system requirements for Lion or ML: A Core 2 Duo or later processor for Lion, and an iMac Mid-2007 or newer for Mountain Lion.