Just brainstorming - I would speak to smaller advertising agencies both traditional and digital and offer them a nice commission to push ads to their client base.

They would expect data proving out the number of impressions/view those outdoor ad spaces will garner and pricing that aligns to the going market rate.

I help a lot of people with go to market strategy and initial revenue generation. Please let me know if you want to do a call to go deeper here and discuss other ideas.