The sources of documentary information which can aid the process of locating literatures relating to selected research topics are:
1. Subject Catalogues of Libraries: These catalogues furnish lists of books available on various subjects. Subject index in libraries also serves the same purpose.
2. Documentation Services: These are a number of documentation centres attached to national institutions. Some of the important centres are:
From these centre codified information on any topic can be obtained on payment of a nominal fee. In addition Xerox copies of pages in books or journals can also be obtained from these centres.