Once you receive your liquid nitrogen rental you will need a source to supply you with the liquid nitrogen. AirGas is the easiest answer because they have thousands of locations, but they can be expensive and cumbersome to work with. The cheaper and less hassle alternative is probably your local welding supply store. Technically, you don’t need to know how to fill these because they will do it; however, it’s nice to know what to expect and make sure they do it properly. If you get a worker that doesn’t care or know what he/she is doing, then they might fill it as fast as possible, put the neck cork or withdrawal system back on it and call it good. This is wrong. The reason is that you need to let the liquid nitrogen dewar cool down or you will get unnecessary boil off.