Is there a database for what is being advertised where?

I am an artist conducting research on billboards advertising legal services. I was curious if there is a database of billboard locations and/or a way to search for specific types of advertising (ie persona attorneys). Right now I’m focusing my search on legal billboards in the Northeast but plan to travel throughout the country documenting them over the next year. Any tips or advice would be much appreciated, at the moment I’m randomly driving around just hoping to stumble upon them and I assume there must be a better approach. Thank you for any tips or suggestions.