Start by focusing on countries whose imports to the United States are granted favored status. This means lower import duties and lower cost for you.
Once you've selected countries as likely sources, contact trade representatives at the appropriate embassies. They should be able to provide you with lists of manufacturers of the products you're interested in.
Attend foreign and domestic trade fairs where companies seeking to export to the United States are exhibitors.
Read U.S. and foreign newspapers and magazines, scanning for advertisements and articles about products you might want to import.