Systematic Reviews (Undergrad): Grey Literature
Search or browse organizational websites (such as WHO, NIH, Robert Wood Johnson Foundation, etc.). Organizations produce reports, statistics, and other unpublished resources. Search sites individually and/or browse areas such as "publications" or "statistics."
Conference proceedings can be a great source of current research, and you may choose to include them in a systematic or scoping review if there is enough information for your analysis. Some databases, such as PsycInfo and Web of Science, include poster and presentation abstracts from conferences. Institutional repositories are also good sources and can be searched through Google Scholar.
Preprints are preliminary reports of work that have not been certified by peer review.