The most complete current source of information is the National Directory issue of Rugby, usually published in February of each year. If you subscribe to Rugby you will automatically get one free. If you don't subscribe then you can order one from
The Rugby Press Ltd. 2350 Broadway New York, NY 10024 Phone: 212-787-1160 Fax: 212-595-0934
(The 1996 advertised cost was $12 for this directory.)
Beginning in 1996, this directory has been combined with that formerly put out by USARFU. The USARFU directory didn't used to list non-dues paying clubs and clubs in independent unions (which meant quite a lot of clubs were not included). It did though contain a lot of useful information about US rugby so the combined directory is extra value.
I am in the process of compiling a searchable on-line list of clubs. It is not yet operational but I'm working on it and am interested in comments.
See also scholarships.