The practical process for starting a country is to control a piece of land militarily, declare yourself independent, and then persuade other countries to recognize your claim. They're incredibly unlikely to do so, unless you can really back that claim up with something. Even countries which are large, have functioning governments and reasonably powerful militaries can't be guaranteed universal acceptance, e.g. Taiwan.
Nobody will sell you land on which to start your own country. Your best bet for land not currently subject to any territorial claims would be Marie Byrd Land:
http://en.wikipedia.org/wiki/Marie_Byrd_Land
a Mongolia-sized slice of unclaimed land in Antarctica. You'd still have a tough time getting anyone to accept your claim, though. Also, it's cold.