Midnight Science, a great resource for crystal radio enthusiasts, was selling single and dual gang ones for $11 and $22 respectively.
http://www.midnightscience.com/catalog5.htmlNot cheap, but if you have to complain then you need to develop your old-radio scavenging resources. I have a few old air-variables I am saving for special occasions. (Right now they are in a low-power transmatch.)