Greenleaf G et al. PhysDoc - A Distributed Network of Physics Institutions Documents. The Query Routing and Forward Knowledge Approach . A Distributed Architecture for Resource Discovery Using Metadata . D-Lib Magazine , June 1998.
Some websites provide catalogues of indexes organized and sometimes ranked.
If the subject you are seeking has a virtual library website, that it is, probably, the best starting point. Irregular about that criteria. Each section maintained by a different expert. They offer the possibility to create your own personal portal. These are mainly lists and databases of search engines, with some indexes also included.