The literature review should be as much comprehensive as possible and for that, one has to be thorough in their research because it is important to include as many relevant resources dealing with your topic of interest.
An electronic database is the first place to begin your search in. Your department at the university may also have archive material, audio-recordings, and video. These can form a substantial part of your research. Take the help of the librarian for more resources. If you miss out on referencing some important research article then your literature review will show weak scholarship. So being comprehensive in your search is rewarding.
Be thorough in your research. Use different keywords relevant to your niche while searching through a database. The database sources are plenty such as Google Scholar, Medline, JSTOR Search, Scopus, and more. While reading a paper you must pay attention to the other relevant papers or book chapters that have been cited. You may be surprised by the different resources that are listed on them, and they could be very broad or narrow in scope. Reading a review article on a published paper is even more useful. Review articles contain a long reference list and evaluate most of the references in it.
Choosing a database can be overwhelming. To overcome that, read the descriptions of the database in your research area. Or you can seek the help of your university librarian on appropriate databases for your topic of research. Sometimes you may need o search in more than one database. Although a database is pretty comprehensive containing thousands of journals it may not contain all the journals relevant to your research field.
Do seek out for research references outside the databases in government website, a website by different professional organizations, different research groups or different think tanks.
If you find absolutely nothing in literature research on your topic, then that might suggest that you are studying a very narrow subject that no one is interested in or it is too novel in your field of work and you are really ahead of the curve. In the latter case, you can look of research that is closely related to what you wish to work on. In that case, you have to cast a wider net but always looking and skimming for topics that are relevant and pertinent to your field of work.