OK. I did some research and spoke with Jenny at AirVoice.
AirVoice Wireless is a MVNO who has a contract with Western Union to sell top-ups for them. The Western Union agents may, or may not also sell AirVoice SIMs. To find an AirVoice dealer you need to call +1-888-944-2355. At this time there are no AirVoice Wireless dealers in major US airports. AirVoiceUnlimited is a new service and the website is a bit underdeveloped.