Preprints are manuscripts that authors make publicly available prior to becoming a formal peer-reviewed publication. Here are some sources of preprints:
Google can help lead you to grey literature. To make sure you are finding high-quality information, I recommend that you use Google Advanced Search. Limit your search to domain types such as .org, .gov, or .edu. Limiting your file type to PDFs will help you get to research papers and fact sheets.
"Grey literature" refers to sources of information that are not published through traditional avenues such as books, scholarly journals, or news articles. Grey literature is often featured on a government website or an organization's website and may include but are not limited to: policy papers, fact sheets, statistics, pre-print articles, clinical trials, and conference papers. These sources often supplement the research you will find from books or scholarly articles.