I'm not chili but...
You can find nitrogren at a lot of places, such as some home brew shops have a drop off where they will have your tank filled, all welding supply places will have it and then there is beverage services that go around picking up and exchanging tanks to local bars and restaurants. Just make sure it's for food use. I've been told that some contains oil in the tank, but I'm not sure the guy knew what he was talking about.
If you buy a tank off ebay or craigslist get the cheapest one, don't get a pretty one because almost all places exchange and you'll never get your shiny new one back, that is to say almost none will fill "your" tank.