Liquid nitrogen is nitrogen gas (N2) cooled and under pressure. Often used in making ice cream and other simple science experiments, it is a dangerous chemical that can lead to death if mishandled.
You can buy liquid nitrogen from a welding or gas supplier. Call ahead to determine the size of a container needed for storage since most suppliers will not sell small amounts of liquid nitrogen.
Cryogenics Equipment Dealers
Cryogenics labs use liquid nitrogen to preserve tissue and to oxidise fuel. Cryogenic suppliers are available online; however most will sell only to institutions. For public use, contact the chemistry department at a local university or a hospital for purchase or give away.
You can compress air and cool it in a metal tank to make your own liquid nitrogen using materials from a home improvement store. But producing it incorrectly can lead to suffocation or frost bite.