I don't know exactly where you could buy it(because I never have), but it should be relatively easy to come by some copper wire mesh(pretty fine wire). Check some electronics catalogs or maybe radio shack. If you put that between the transformer and the monitor or wrap the transformer in it, it should shield against the magnetic field relatively well(probably enough to get rid of the distortion). The other alternative is to try to buy a transformer that is already shielded, but that would probably be more expensive. moving the transformer out of the cabinet is probably a little drastic, but if nothing else works, you might have to.