Liquid nitrogen is available from many industrial suppliers as well as from online retailers. For example, Praxair in the United States sells compressed nitrogen gas and liquid nitrogen in various quantities.
Most of the expense in obtaining liquid nitrogen doesn't come from the gas itself but from the specialized dewars needed to contain and store it. A thorough knowledge of the characteristics of the gas and liquid are necessary because a small amount of liquid nitrogen sublimates into a large amount of gas. This can cause inappropriate containers to explode. The flasks must be specifically designed for holding low-temperature fluids.