Here's how to find a carrier. Google up "commercial insurance brokers" for your area, and call them. Anyone I know will be in Texas or Missouri, and they won't be licensed for New York. You will not find anyone who understands this coverage from the State Farms and Farmers Insurance type of companies.
The policies are held by unusual sounding groups like American Modern, and only a commercial borker has access to them.
I would also call the other, smaller, billboard companies in your area and ask who they use. At least a few will tell you. Even then, I'd bid it through a commercial broker, as that person may not have the best pricing on their policy.