I've mentioned in the past that I often look for postcards in the gift book section, on the front counters, in the humor section and in the art section... I'd like to add another section to the list - The movie book section. Not the section where they sell movies, but the section where they have books about movies. That is where I found the Marilyn Monroe cards.
I'm not sure if I have any Postcrossing people out there who still read, but I'm looking for blank offbeat/offensive postcards. So if you want to trade a few cards please let me know! Also, a fun source for fun postcards (though they tend to cost a little more than your typical used bookstore find) - Chronicle Books.