You are going to narrow down what industry you are actually looking to rank for, the other issue you will run into is obviously the GMB pages. However, once you have all of that sorted its just a matter of digging around looking at the search traffic and finding what is a good target market, but not hyper-competitive. Don't try and go for NYC locksmith or something insane, look for smaller cities that have decent search for service businesses.
Once you have the site ranked you can then call local businesses and offer to sell them the leads or just sell them the entire project.