Is this just a specific kind of centrifuge used only for nuclear material?
Short answer, yes.
And it is not commercially available? Why do they have to get them from rogue scientists? If they have the technology to make nuclear weapons, how do they not have the technology to design a device that rotates at a high rate of speed? They have jets, don't they? Can they build a washing machine? If they can, they can build a centrifuge. Hell, you can even soup up a washing machine and use it as a centrifuge. I just don't get what the significance of the centrifuge is.