In Pinellas and Hillsborough County, virtually all the billboards are owned by two companies, Clear Channel Outdoor and CBS Outdoor. That becomes less true when you get into some of the outlying areas, such as going up US 19 into Pasco and Hernando Counties.
If you’re looking for boards in Clearwater, for example, you have only two companies to call.
In other areas, sometimes the only way to find out who owns boards is to drive the area and note down names and phone numbers off of the boards.
An important term in billboard rental is “avails”, short for “availables.” The fact that a board has or hasn’t got advertising on it tells you nothing about whether or not it is available next month or not till next year or whatever. So first narrow down where you want to be placed, then see what is available, then see if there’s anything suitable for when you are looking to start your campaign, and at a price that makes sense.
Sometimes you get surprised. Don’t try and rent a billboard in Springhill for example. There aren’t any.
Also, you can’t decide to advertise today and have a board up tomorrow. Your ad has to be designed, printed, and put up (which can’t be done when its raining). So figure a minimum of 3 weeks from when you say “go” to when your board goes up.