This is all predicated on the assumption that they're not being supplied with what they need. I am opposed to all UN Inspection programs. The reason? They kept telling us in Iraq how their entire WMD program could fit into a trailer. How can you inspect a whole country? Any nation that wants to keep the inspectors in the dark can do so.
Nuclear programs are much harder to hide because they require large, complicated, and delicate industrial equipment.
It is theoretically possible that Iran would be getting HEU from another source. Two things, though: (1) if they could buy HEU, why go to the trouble of making it? and (2) if there's someone out there selling weapons-grade uranium, we have a much bigger situation on our hands.