Tatjana Buisson, a designer and photographer who loves postcards, has started a website called Postcard Happiness, which is dedicated to encouraging people to send more postcards. She also began a Kickstarter campaign in order to raise money to manufacture and market something called a Postick.
Posticks are white, adhesive labels with postcard-backing lines printed on them. You can use them to convert anything (roughly the size of a postcard) into a postcard. When you start to think about it, you'll probably find that you have lots of potential postcards lying around--you could stick a Postick onto a photograph, a drawing, a newspaper clipping, part of a map, a vintage postcard, a piece of fabric, etc. I love the idea of giving something that would otherwise be stuck in a drawer, or even tossed into the garbage, a second life as a postcard.