Be careful not to box yourself into a concrete search term for your topic. Think of synonyms for your subject - broader terms, related terms, and narrower terms. Using this strategy will guarantee a wider selection of materials. Be aware, too, that some search terms may be uncomfortable to think or say out loud - but could yield possible results.
Possible search terms: Immigrant, human migration, refugee, undocumented person, illegal alien, émigrée, displaced person