Clinical guidelines can also be a source of studies. In relation to the hierarchy of evidence, clinical guidelines provide high quality summaries of information. They are systematically developed to assist health professionals make clinical decisions about appropriate health care.
Authors of clinical guidelines have appraised existing research in order to translate the evidence into actionable recommendations.
A few key sites are listed below. Use a search engine to explore further.
'Specify all databases, registers, websites, organisations, reference lists, and other sources searched or consulted to identify studies. Specify the date when each source was last searched or consulted' - PRISMA 2020 Explanation and Elaboration, p. 5