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.
Econlit includes references to working papers from a number of research organisations. NBER Working Papers contains the full-text of the National Bureau of Economic Research's working papers - these are not freely available online.
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'.