• World's Columbia Exposition, Chicago Illinois, 1893

• California Mid-Winter International Exposition, San Francisco, CA  1894

• Cotton States & International Exposition, Atlanta, GA  1895

• Trans-Mississippi & International Exposition, Omaha, Nebraska  1898

• Pan American Exposition, Buffalo, NY  1901

• South Carolina Interstate & West Indian Exposition, Charleston, SC  1901-02

• St. Louis World's Fair, St. Louis, MO  1904

• Lewis & Clark Exposition, Portland, Oregon  1905

• Jamestown Exposition, Jamestown, Virginia  1907

• Founder's Week, Philadelphia, PA  1908

• Alaska-Yukon-Pacific Exposition, Seattle, WA  1909

• Hudson-Fulton Exposition, New York.  1909

• Panama-Pacific International Exposition, San Francisco, CA  1915

• Panama-California Exposition, San Diego, CA  1915-16

• Sesqui Centennial Exposition, Philadelphia, PA, 1926

Look for an Exposition Station cancel on the stamp side of the postcard - this shows that the card was actually mailed at the Exposition, from the Exposition post office.  The Jamestown Exposition was held in Jamestown, Virginia, but the Exposition Station cancel reads, Norfolk VA.  Look for this cancel on the 1 cent green John Smith stamp.  The Panama-Pacific Exposition of 1915 and Sequicentennial Exposition of 1926 called their exposition stations a "Model Post Office."  These Exposition cancels say Model Post Office. 

Exposition Postcard Links:

Postcards of the Louisiana Purchase Exposition, 1904.  Collection of exposition postcards.

Penny Postcards from the Panama-Pacific International Exposition, 1915.  Collection of postcard images.