International Buyer Program
The U.S. Commercial Service's International Buyer Program (IBP) brings thousands of international buyers each year to meet with U.S. companies at major trade shows in the United States. Each year the U.S. Commercial Service selects and promotes 28 trade shows representing leading industrial sectors.

Listed by industry:
http://www.export.gov/eac/trade_events.asp

Listed in date order (U.S. fiscal year)
http://www.export.gov/comm_svc/ibp_events.html

Know which U.S. city, but not the date of your event?
The web site of the local Convention and Visitor Bureau can help you... follow the State and City links via the portal below:
http://www.seeamerica.org/

Search engines are another good source of information on trade events:

TSNN - The Ultimate Trade Show Resource
http://www.tsnn.com

Trade Show Exhibitors Association
http://tswstg.cahners1.com/index_tsea.asp