Unless you are familiar with a particular online reference resource that you know will provide the kind of information you need, it's probably most efficient to refer to a reliable online reference resource directory. These directories may be subject-specific (Tarlton's reference links) or interdisciplinary (UT General Libraries' reference links). Commercial search engines often include web directories and those directories almost always include a reference classification.
Tarlton Law Library reference information
UT General Libraries online - databases and indexes to articles
Google (and other search engine) reference directories