Atomic clock devices are expensive peices of equipment mainly confined to national laboratories. However, there are a number of atomic clock references available free to air for computer time synchronisation.
The GPS satellite system has on-board atomic clock units that can be received and utilised to synchronise computers and computer networks. The atomic clock GPS system is available world-wide. Any GPS antenna with a good view of the sky can synchronise with the GPS satellite system.
Utilising the GPS atomic clock system, much finer accuracies can be achived. The GPS atomic clock system can provide timing to within a few hundred nano-seconds of UTC.
Galleon Systems NTP Servers and Radio based time servers have GPS reception options.