Using index terms is your best bet to find articles in specialized databases. However, sometimes we will also want to search for keywords from the title, abstract, and author-submitted keywords of the article as well. 

General academic databases, such as Scopus and Web of Science Core Collection, do not tag articles with index terms when they add new records.

Additionally, very new articles might be added to specialized databases before they have been indexed. 

Many of the thesauruses have 'synonyms' or 'entry terms' that they suggest using as keywords to search with as well.

Other places to look for text words include the abstracts of articles that you have already identified, relevant textbooks, or from public facing websites produced by major academic and educational institutions.