Background or "reference" sources such as scholarly encyclopedias, handbooks, and bibliographies are a great place to start. They provide good overviews of the relevant scholarship and always include citations to secondary sources. Often they include primary source suggestions too.

The SHAFR Guide Online: An Annotated Bibliography of U.S. Foreign Relations since 1600

Latest edition of the major reference work for the study of U.S. foreign relations. Chapters are arranged both chronologically and topically. Among potentially relevant chapters is Ch. 28, "Economic Issues and U.S. Foreign Relations." 

Cambridge Histories
A key reference collection. Relevant works here include The Cambridge History of the Cold War and The Cambridge World History, Vol. 7, Part 1 and Part 2.

The New Palgrave Dictionary of Economics

For technical references on economics issues.

Oxford Research Encyclopedia in American History

A fairly new resource that is likely to have relevant articles such as "United States Foreign Economic Aid."

Oxford Bibliographies

A large collection of annotated bibliographies, often with citations to both primary and secondary sources.

Oxford Handbooks Online

The Oxford Handbooks may be a helpful starting point for any number of topics. Books in this series include The Oxford Handbook of the Politics of Development.

Gale Virtual Reference Library

Includes the International Encyclopedia of the Social Sciences and other sources useful for quick references.

Government information and related sources:

• ProQuest Historical Newspapers via ProQuest News and Newspapers

  Approximately 55 mostly U.S. historical newspapers with coverage from the 19th to early 21st century. Includes the New York Times and Washington Post.

   
• Gale Primary Sources

  Includes The Economist and Financial Times historical archives.

   
• International Herald Tribune (1887-2013)

   

   
• World Newspaper Archive

   

   
• International Newspapers on Microform in the Yale Library

• FAS Foreign AID CRS Reports (mostly 2000s)

  Selected CRS reports- policy documents for Congress that give overviews of the policy

   
• EveryCRS Reports- Foreign Aid Search

  Search for foreign aid in EveryCRSreports... provides resources which say back to 1976

   
• Congressional Research Reports

  Search for current foreign aid documents in this Congressional portal. Not only will this provide the CRS report, it will also link to versions of each reports if there have been edits.

Use the Intergovernmental Organization Search Engine (a Google custom search) to search across hundreds of IGO websites:

Use the Non-governmental Organization Search Engine to search across NGO websites. Sites were chosen based on their consultative status with the United Nations Economic and Social Council (ECOSOC) and also collated from University of Minnesota Human Rights Library, Duke University Libraries' NGO Research Guide, and the World Association of Non-Governmental Organizations (WANGO).