I spent ages trying to track down n reg operators on the internet and it is not an easy task. That link to the registry list is not really much help as most are owned by a trust and give no indication of operator. One way i found that was not too bad was doing airport specific searches on airliners.net and often the photographer has gone to the trouble of finding out the operator so they can add it to the photo information box. A benefit of the slightly ocd nature of the spotter.