It is almost impossible to approach national advertisers with single billboards. They normally buy signs on a national scale, in many markets from one billboard company. You need to focus on local advertisers, who do buy signs one at a time. You have the same competitive advantage with local advertisers as the biggest companies do – in fact, you probably have an advantage over them, as they normally have lousy customer service and rates that are too high.
That is not to say that you can’t keep ad agencies (that’s who places the ads for national companies) in the loop. I would send them your availables every month as a courtesy. But local advertisers are going to be your bread and butter.