McG -

You can use ordinary air, but you don't compress it. You chill it.

The nitrogen liquifies a fair bit before the oxygen does, so the first bit after you hit about -190F¹ (-90C) is pretty pure nitrogen. Just pour it in a can and let it evaporate.

(Don't stand too close to the can while it evaporates, if ain't got a real loose lid.)

¹ From memory with no guarantees. I ain't gonna look up the exact temps for you.

John