I've been on a postcard buying kick this year (mainly because I've been to museums that sell good ones!). Here are a few I wanted to document before I send them off into the mail. Perhaps you will be receiving one of these someday soon.
Maurizio Cattelan, Daddy Daddy, 2008 installed at the Guggenheim (Purchased here).
William Eggleston, Las Vegas, c. 1965-68 (from one of the best postcard sources and location of the following three images)
Consumer Reports Tests Shampoos, 1955
Neil Winokur, Glass of Water, 1990
Tom Wesselmann, Still Life #30, 1963 (bought here)