Land can be bought or sold on the land page under the "My Nation" dropdown menu. The cost of land is exponentially related to the amount of land you already have. The key thing about land is population density - there is an optimum population density at which your nation will experience its highest growth rate. A lower or higher population density will negatively affect your birth rate and death rate.
One billion is currently the total land available for all nations in Project Terra. The cost of land will also increase as total available land decreases.