Go to Business Source Complete.

• In the Publication Type field, select Working Paper.

Go to EconLit, which indexes working papers. 

• In the Source type field, select Working Paper. 

Use Social Science Research Network (SSRN).

Google Scholar is a great resource for finding working papers.

Also, look at Technical Reports and more at the Science and Business Reading Room from the Library of Congress.

Other resources include:

• Asian Development Bank Institute Working Papers Series.
• Centre for Economic Policy Research (CEPR) Discussion Paper Series.
• Columbia International Affairs Online.
• Federal Reserve's Finance & Economics Discussion Series.
• Federal Reserve's International Finance Discussion Papers.
• IDEAS at RePEc (Research Papers in Economics).
• National Bureau of Economic Research (NBER) Working Papers.
• Policy Commons.
• Rand Corporation.