There are comprehensive, (although not exhaustive) lists exist as a one-stop-shoo to discover sources, agencies, and organisations in one place without the need to surf around the Internet to discover piece-meal. There are inevitable on-going changes given the nature of the work but can hep people to discover and learn more on the subject. However, even some of those who change locations or links will at least be able to re-direct you to other contacts known through historical networks.