I buy mine from a local welding supply. I buy it under a business name and they made me sign some hazmat document.
I found a used 50 liter dewar on ebay for around $100 (a real steal!). It was some university surplus.
I made a lid for it with a pressure relief valve as well as a valve and a draw tube for dispensing. Works great!
Basically a poor mans version of this: http://static.colepa.../0377367APP.jpg
Here is a pic of a similar one: http://www.ebay.com/...=item5d4f787d3c
I don't fill it all that often, mostly for making LN2 ice cream, which is awesome!