"The Maritime Administration (MARAD), in order to gather as much information as possible about national flag preference, selected 56 countries as a basis for a detailed study of the nature and kind of national preferences and their legal provisions. [...] The governments of the selected countries contacted and asked to provide information regarding the existence of flag preference laws. Fifty-three countries responded to MARAD's request." This document contains a compilation of the responses of these countries in table format.
U.S. Department of Transportation, Maritime Administration: http://www.marad.dot.gov