Interactive AM Pattern Map

Here's a nice tool for finding a blank AM frequency from Radio-TimeTraveller Blog.
It's called the "US Mediumwave Pattern References", produced by the Radio Data MW program. It provides a visual representation of the most current signal patterns of all licensed AM stations in the US and Canada.

You simply select to view the daytime or nighttime patterns then pick a AM frequency.

For part 15 AM broadcasters, a general rule of thumb is that the higher the frequency you broadcast in, the better the range, and since nighttime signals of Part 15 generally perform poorly, I selected 'Daytime' and '1700KHz'..

So as long as you are not located within the patterns displayed, then this shows that 1700AM would be a good candidate as a frequency choice for your part 15 station.

Don't want to broadcast that high? Select a different frequency.

You can make use of an online collection of the database here: http://www.nf8m.com/patternmaps.html Start by selecting Daytime or Nighttime

And/Or download the complete set of results for all frequencies (530-1700) here:

US-CA Daytime Pattern Reference 530-1700 KHz (01-22-2015)

US-CA Nighttime Pattern Reference 530-1700 KHz (01-22-2015)

EDIT: 2/9/2018: The two links above no longer work, here is his updated version files for offline use: http://www.mediafire.com/file/9uhyas1jxw7kxux/2016+US-CA+Pattern+Reference.zip

HOW THEY ARE PRODUCED
Using the actual FCC database files Radio Data MW will auto-generate an interactive HTML pattern map, showing the pattern plots for all stations included at the discretion of the user. A complete set of mediumwave pattern maps can be generated within minutes. Radio Data MW generates a real pattern plot based on ground conductivity, ground dielectric constant, and can display actual (but approximate of course) signal level boundaries for Local, Distant, Fringe, Extreme mV/m levels, or any custom mV/m level chosen by the user.
See this post from Radio-TimeTraveller Blog for more details.