Working papers (also known as discussion papers) are pre-publication research papers which tend to be made available by the author’s employer, e.g. universities and research organisations. They are usually written at a relatively early stage of research and are often published formally as journal articles when the research is complete.
Many universities and research organisations make their working papers freely available on their websites. If you are aware of a University department, research centre or think tank that is doing research in your topic area, try their website. Many websites list working papers under 'Publications'.