Well this a great question actually. I'm willing to bet a lot more people are wondering, but don't ask.
Dewars can be bought a bunch of places. You can find deals on forums, find them on ebay, and I believe Airgas sells them (more on Airgas later). Most recently though, there is a place called "Blackholesurplus" that is based in New Mexico and sells dewars on the cheap. (Link:
http://www.blackholesurplus.com/ )
When looking for a dewar, what you want to look for are three things:
1. The most important is that the dewar still has a vacuum. Since LN2 is
very cold, it would simply evaporate if allowed contact with the outside air. Therefore, dewars are simply double walled metal containers. To keep the inside wall cold and the outside wall warm, the air is pumped out from between them (creating a vacuum). If this vacuum is still good, then the dewar will hold LN2 for weeks (if not months) on a single fill.
2. The dewar size is important as well. Depending upon personal needs, 30L will do just fine. It's easy to overshoot the capacity on these thinking they will be small. Even a 30L dewar is pretty large. A 50L is right on the line of "uncarryable." Choose wisely on the size because getting too large will cost you.
3. Extraction method is the last important part. There are two ways to get the LN2 out. You can have a "tip 'n' pour" type dewar which simply has a hole at the top from which you'll pour the LN2. The other type is a pressurized canister. This can be dangerous if the safety valve isn't working, but is generally much easier to use. Basically the contact are under pressure and you have a spigot (like a hose almost) which you turn a valve on to release the LN2. These, being much easier, come with a hefty price tag.
Next is your LN2 supplier. Depending upon where you live, it can be harder (or easier) to find LN2. A bunch of common places that
use LN2 would be Hospitals and Schools (more likely colleges). So if you're near those, an LN2 supplier usually will be.
Airgas is a nation-wide chain which sells DIce, LN2 and other pressurized gases. (Link:
http://www.airgas.com/ )
For a list of their outlet stores, use the "Store Locator."
Another way to find a supplier by you is to simply google "Liquid Nitrogen near [insert postal code here]."