I can get replacement needle and nozzle for my Iwata HP-CS airbrush at very reasonable cost at my local Hobby Lobby, and save even more using their 40% off coupons.
You cannot easily get replacement parts for the knock-off airbrushes. And for the very few that you can, the needle plus shipping is 2-3 times more than the Iwata needle. Since the knock-off uses cheap materials that don't last that long, it is sensible to plan on getting complete brand new set instead of replacing parts.
But those disposible may make perfect sense for people who only airbrush occasinally and don't want to invest in a name brand upfront.