Clinical guidelines and position statements can be found in many different places. The National Guidelines Clearinghouse was the "go-to" source until July 2018 when funding ceased. Professional organizations (e.g. medical/nursing specialty, disease/disorder), a few of which are listed below, often have guidelines. Many times they are available for free, but there are some organizations that require a membership in order to access content from their website.
A comprehensive list of nursing organizations (National, State, International) is available from the nurse.org website.
Practice guidelines and clinical guidelines are available from a wide variety of professional organizations.