Nitrogen is the major constituent of the air we breathe. When it is condensed from the air, it is an extremely cold fluid at -320 degrees F(-196 degrees C). If you've never handled liquid nitrogen before, I'm reluctant to recommend that you try this experiment. Liquid nitrogen can cause serious damage to skin and eyes if it is improperly handled. While it is relatively cheap to purchase, you can't store it for more than a couple of days. The storage containers can be quite expensive. If it is improperly stored, the storage container can explode. Obviously, this project requires responsible adult supervision.
Now that you've been warned, here's how to handle liquid nitrogen. You will need a Thermos bottle (the real glass vacuum bottle kind) to store the nitrogen, and goggles and insulating gloves to handle it. Actually, to make ice cream, you'll need at least 3 or 4 liters of liquid nitrogen, so you may need several bottles. If you have a few hundred dollars for equipment, you may want to buy a 4 liter dewar from someplace like Cole-Parmer.
If you use household-type Thermos bottles, get the largest you can find. Drill a 1/4" (6 mm) or larger hole through the lid to allow the vapor to escape. DO NOT under any circumstances ever put liquid nitrogen in a sealed container, because the container will explode.
You will find that a small Thermos will hold nitrogen for about a day. Larger bottles will do better. This is normal, because the nitrogen continually evaporates even in the Thermos. That is why the lid must have a hole in it. Don't let the hole get plugged by ice condensing from the air!
If you spill liquid nitrogen on your clothes, you will probably be 'burned'. (Your skin will be frozen where the fabric held the liquid in contact with it.) Small amounts of nitrogen falling on bare skin usually skitter off before causing a serious burn.
You can buy liquid nitrogen through welding supply shops. Look in the Yellow Pages under "welding supplies" or "welding equipment and supplies". Be forewarned that they may not want to sell small quantities (less than 50 gallons), so check before you spend money on containers. If there is a college near you, they may be able to give you some. Call the college's Physics Department.
Good luck!