While deciding on a topic, you will probably be finding studies that address your research question. These are very important in creating your inclusion and exclusion criteria and your searches. If an article that you found via any means is something you would include in your review, then you need to be sure that your searches turn up that article. The best way of making sure of that is to use the example articles to find search terms that will also turn up additional studies that you haven't found yet. A subject librarian can assist you in creating searches based on example articles.
Choose important databases in your subject to search. You may also want to choose a broad-coverage database (such as Google Scholar, Microsoft Academic, Dimensions, or the Lens) to discover interdisciplinary works related to your topic. It is particularly important to do very deliberate and structured searching, so getting familiar with the advanced search options and with "command line" searching can be very valuable. Your Subject Librarian can provide advice on this and other aspects of searching.
Note: there is a certain aspect of "what we have available" that drives database selection. If we do not have a particular database, it may be worth the trouble to visit another institution (or work with a colleague) with access. Librarian Rebecca Hedreen can help if you are not sure about visiting another institution.
Besides your regular subject related library databases, consider some of these specialty search sites:
Grey literature is any sort of the publication beyond the standard "books and journal articles" of scholarly publishing.
It can be important, depending on the extent of the review, to include grey literature. However, grey literature is notoriously hard to search for, making a session with a librarian nearly required.
In some cases, citation tracing (looking at references and articles citing already identified sources) can be used to add more materials to a review. This is a separate process from searching and should be reported separately.