WorldCat searches hundreds of online library catalogs across the US and world. Public libraries are also helpful in distance education activities. If you already belong to a local, public library, you should be eligible to use their interlibrary loan services to obtain books, articles, and other items from libraries across the country. They may also have access to other libraries' catalogs and other resources unique to their customers. So, don't forget to check out the resources and services at your public library too.
Indexes are akin to subject lists of articles.
Also, while some indexes are available online, others are only available in good old fashioned print format inside a library building. Indexes remain the best tool to use in order to find articles by subject or within a particular discipline of research. Each library usually maintains a catalog. Therefore, a library catalog is unique to each library. However, periodical indexes may cover entire research fields.