Given the number of other sites which appear to have access to a more complete and accurate copy of the routing registry data, am I looking in the wrong place or do I need to pay/register to get accurate data?
Não, você não pode obtê-lo, a menos que você se enquadre nas qualificações para um Whois em massa:
ARIN provides access to a bulk copy of all objects in the ARIN Whois directory service to support the work of bona fide academic researchers, and to operators and researchers who are using the data to provide a clear benefit to the broader networking community. ARIN does not provide bulk copies of Whois data to operators who wish to incorporate this data into products, services, or internal systems with no clear benefit to the broader community.
O que os outros sites estão fazendo é consultar a API do ARIN