The wonderful REC Networks (one of the unsung heroes of the low-power radio movement) have developed tools to research the availability of low-power radio opportunities in neighborhoods across the country.

For the tech geeks: here is the beta tool to goof around with:
http://mylpfm.com/search/?

For the rest of us: here is the more stable tool until the beta version if perfected:
http://cdbs.recnet.net:8080/lpfm.php