First of all, Mountain Lion is only available through Apple's Mac App Store. And, Apple's Mac App Store can only be accessed on systems running Mac OS X 10.6 Snow Leopard. Why? Because that's the version of the OS that includes the App Store app, and the App Store app can't be installed otherwise.
So, if you're running Leopard (10.5) or something older, an upgrade to Snow Leopard is required, as are an Intel processor and 1 GB of memory. Simple enough. Just download and install it, and we're off to the races. Right? Wrong. Snow Leopard can only be installed from a disc, a disc that Apple no longer offers. "It has to be obtained from a third party," said the Apple Genius. Apple recommended obtaining the disc from Amazon or CDW, which we did. Two days later, a box with a smile on it was at our door. Following a Time Machine session, Snow Leopard installed without a hitch.