Nitrogen is a very common gas, making up ~ 80% of the air we breathe. Nitrogen is cheap and readily available as it is widely used in industry, catering and occasionally in home beer brewing. Nitrogen cylinders can also be stored indefinitely in the cupboard for possible future use.
Sources of Nitrogen US
In the US, full steel cylinders containing 550 liters/ 20 cu ft of Argon can be ordered from a company called CyberWeld, and are delivered nationally.
Note: CyberWeld ceased selling full tanks of nitrogen in mid 2019. Although argon is as effective as nitrogen, it can be detected at autopsy.
In Australia, suitable, take-home cylinders of nitrogen are available from Stuggots and from Total Tools. The advantages with ordering from Stuggots include: shipping is included and confidentiality.