A professional organization (sometimes called a professional association) is typically a non-profit group that professionals in a specific field of study may join to gain new skills, share skills, network with others in their chosen fields, learn about new training opportunities, etc.
Many professional organizations have trade magazines or publish academic peer-review journals and can be a great source of information.
To find a professional organization, you can always use a search engine like google to google: Profession + Professional Organization or use the tool Professional Association Finder from careeronestop.org below.
While not a scholarly source, newspapers and magazines can have relevant information depending on your research needs. Sometimes they can even serve as a great primary source.