Because it makes up 80% of our atmosphere, Nitrogen isn't hard to find, but to turn it into its liquid form, you might need some high-tech equipment. The process begins when air is compressed in a compressor. What happens is the air molecules are pushed closer together, and then shoved down a tube, and into a chamber. For the gas to expand, and as the gas expands, the molecules need more energy, and they get this energy by absorbing heat from the chamber. The process is repeated until nitrogen begins to condense into its liquid form. A funnel can be placed at the bottom of the chamber to collect the really cold liquid nitrogen.
Or you can just go out and buy a huge container full for about 20 bucks, only problem would be finding a suitable container to hold it. Keep in mind that when liquid nitrogen is sealed up in a container, and then suddenly released, it will explode just like a bomb.
Personal Experience: At the end of the year, Physics class took a 180 spin and become my third favorite class (behind 2 computer classes). We got to play with the coolest thing ever, Liquid Nitrogen. We were freezing things left and right. Anything we got our hands on went into the cup of liquid nitrogen.