You can only buy OS X Mountain Lion through the Mac App Store, which again only runs on OS X 10.6 and above.
If you can get hold of any Mac, sign up for the App Store, then download OS X for installation on that machine, it will download an
.app file which contains the installation disk image for OS X Mountain Lion.
For OS X Lion (10.7), you were able to buy a USB key with the software. This is not possible anymore for Mountain Lion.
Note that in any case, the license only permits you to run OS X on an Apple computer (and virtualization only when running OS X as a host), and in countries where software licenses are legally enforced, circumventing that may be considered illegal. But IANAL.
You have the right:
to download, install, use and run for personal, non-commercial use, one (1) copy of
the Apple Software directly on each Apple-branded computer running OS X Lion or OS
X Snow Leopard (“Mac Computer”) that you own or control;
to install, use and run up to two (2) additional copies or instances of the Apple
Software within virtual operating system environments on each Mac Computer you own
or control that is already running the Apple Software, for purposes of: (a) software
development; (b) testing during software development; (c) using OS X Server; or (d)
personal, non-commercial use