You can buy a Nitrogen generator. A common method of doing this is to use pressure swing adsorption (PSA) where you run the air through zeolites (crystals designed to adsorb a certain molecular size).
The system pressurized the air, runs in through the zeolites where the O2 is adsorbed and then it switches over to another bank and the first one is depressured which desorbs the O2. The Oxygen is therefore the exhaust stream.
The system alternately switches between several of these banks to provide Nitrogen. Note that you typically do have some residual O2 in the N2 from a PSA system.
If you want 100% O2 free Nitrogen then it is best to cool the air until you reach the boiling point point of oxygen (-183C) and then condense out the oxygen. Obviously this is much more expensive to do. You can buy PSA Nitrogen generators for tens of thousands of dollars but a refrigerated N2/O2 plant will be tens of millions.
The biggest question would be what are you using the Nitrogen for (how important is purity) and how much are you using. That will dictate the best solution.
In most cases it is easier and cheaper to buy a Nitrogen bottle but there are many cases - for example to feed a CNC laser cutting machine - where you could justify buying your own PSA type Nitrogen generator.
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