A journalist friend was kind enough to share some of her expertise in finding experts. Note that the two principal sites require some amount of registration/identification, though both are no-cost. The third is a compilation of source-finding resources. She mentioned:

ProfNet from PRNewswire. That parent insitution is a pretty mainstream public relations "professional communicator" site, so it is likely that the experts will tend towards "mainstream" business orientation. As with any information resource, it is useful to have a sense of where the source is coming from.

HARO - Help a Reporter Out comes out of the social networking/IT milieu, with the feel of being the product of an individual's vision. The countercultural style affected by much of this sector often masks mainstream substance, of course. As always, proceed with eyes open.

Additional ideas can be found on the Expert Sources page on the American Press Institute's Newspaper Next site.