how do i ins.my business?

I have a small tent trailer rental business but i can not seem to find an ins. co. than will offer me special ins.for when my customers rent a tent trailer i know some of the biger outfits offer there customers ins. how do they do it and how can i do it. I am loosing a lot of customers because there ins. co. will not offer them a writer. I do not know what to do. please help.