Some of your search terms will describe your specific topic and some the nature of your interest. In both cases, you need to be flexible, observant, and imaginative. Before searching for journal articles, look for encyclopedia articles or books that might give you an overview of the subject area, describe aspects of it that you were unaware of, and expose you to terminology that had not occurred to you. Exploring an interest in the rhetoric of Japanese gardens, I might discover references to specific types, such as Zen gardens or tea gardens. I would learn about their history and representation in art, from which I would discover the names of significant artists. I would also realize that my search for journal articles should employ terms such as "design," "composition," "form," or "motif" rather than "rhetoric." When I began searching for journal articles, I would make use of databases devoted to art and architecture, history, religion, and philosophy, as well as general humanities and rhetorical studies.
Keep in mind that if you have an idea of what you want, you can just search for specific collections.