Good places to get liquid nitrogen have already been covered by others, but if you're going to use it and have no experience with the stuff then I'd like to put in a few words on safety. Liquid nitrogen is actually pretty benign under most circumstances- sure, if you do something stupid like stick your hand in it you can get a nice case of frostburn, but ultimately touching the stuff is uncomfortable enough that any accidental contact will be brief enough that no serious damage will result (just don't splash it in your eyes... but that holds true for most chemicals). What you
do need to be careful about is making sure that you never keep the stuff in a sealed container without some kind of pressure release. Doing so will result in some serious pressure building up as the liquid nitrogen slowly warms up (and returns to being a gas), and when that pressure ends up rupturing whatever container you're using the results can be http://pipeline.corante.com/archives/2006/03/08/how_not_to_do_it_liquid_nitrogen_tanks.php.
For those not interested in reading the article, here's the good part:
The cylinder had been standing at one end of a ~20' x 40' laboratory on the second floor of the chemistry building. It was on a tile covered 4-6" thick concrete floor, directly over a reinforced concrete beam. The explosion blew all of the tile off of the floor for a 5' radius around the tank turning the tile into quarter sized pieces of shrapnel that embedded themselves in the walls and doors of the lab. The blast cracked the floor but due to the presence of the supporting beam, which shattered, the floor held. Since the floor held the force of the explosion was directed upward and propelled the cylinder, sans bottom, through the concrete ceiling of the lab into the mechanical room above. It struck two 3 inch water mains and drove them and the electrical wiring above them into the concrete roof of the building, cracking it. The cylinder came to rest on the third floor leaving a neat 20" diameter hole in its wake. The entrance door and wall of the lab were blown out into the hallway, all of the remaining walls of the lab were blown 4-8" off of their foundations. All of the windows, save one that was open, were blown out into the courtyard.