I don't think one exists.
I will tell you that my homeowners 'required' I pay for a separate policy to cover my horses on my property or they couldn't issue a home owners policy.
I asked for an explanation of the coverage. This is a grim description but it is what he told me.
If my horse got out and was hit by a car and the person died. The insurance company would cover the death of the person and not my horse.
If the person was injured with long term health issues I could still be sued for it and the insurance company did not cover that.
So my guess is that you would be hard pressed to find a 'reputable' insurance company that would issue a liability policy just for an animal to cover a lawsuit.
Albiet there are 'so-called' ones listed on the internet but I'd bet that when the lawsuit hits the courts those fly by night company's would leave you hanging out to dry.
Of course inquiring minds would like to know why you think you need one on the dog you own? :W
We must be willing to get rid of the life we've planned,
so as to have the life that is waiting for us.