As is the nature of the Web, sites come and go, and contain varying amounts of information. These are the ones I find worth keeping an eye on.
1 Mamiya America Inc. ( http://www.mamiya.com ) used to have a web page with a User Forum for older cameras which included considerable discussion on the TLRs. As of February 2007 this service appears to have been closed. A lot of accumulated lore went away with this service. They can supply manuals for some models (C3*, C33*, C330F*, C330S, C220F (*photocopies)) within the USA only. An online version of the C330 manual in Portable Document Format format is available. A similar facility is provided by Mamiya UK ( http://www.mamiya.co.uk ). The various Mamiya agents have been posting revised history documents in the past year or so that under-play or ignore the Mamiya C series.
2 The new database-supported Medium Format Digest is the Medium Format forum at Photo Net ( http://photo.net/ ).
3 APUG http://www.apug.org is often a good place to ask film camera related questions.
4 There is some useful material on Barry Toogood’s http://www.tlr-cameras.com including a reprint of a 2006 article from the British Amateur Photographer magazine.
Otherwise, try some carefully crafted searches. It often pages to dig beyond the first couple of pages – film cameras are becoming a smaller and smaller niche interest.