You wouldn't necessarily liquid cool with liquid nitrogen - you would pour LN2 into 'pots' that are essentially bolt on blocks for your PC that literally hold liquid LN2 while it boils off. But, I would start at a welding gas supplier and see where that leads you. I would imagine that LN2 actually requires some form of license in order to procure...at least in any significant volume.
You have A WHOLE LOT of reading and info to go through, BTW. This is is a cooling solution for those running very custom BIOS's, custom hardware with modified power delivery systems and for short-term benchmarking only.
http://www.technologyx.com/featured/beginners-guide-subzero-benchmarking/
https://www.overclock.net/forum/268...-sell-dice-ln2-pots-ship-internationally.html
http://www.xtremesystems.org/forums/showthread.php?244422-Buying-a-Hardcore-LN2-Pot
Otherwise, if you're looking for long-term, sub-ambient liquid cooling, you should be checking into some of the more advanced forms of Peltier/TEC liquid cooling systems. 4ryan6 is one of our forum members who has a long-term setup like this, although I think the photos are broken on his page:
http://www.tomshardware.com/forum/282844-29-peltier-water-cooling
https://www.overclockers.com/forums...ltier-TEC-Assembly-Now-17-Months-in-Operation
Alright thanks so much for you help. I really appreciate it