Taxi Driver Online

Does anyone know of a company that looks for clients/business's that want to advertise on the side of a taxi or are we better scourcing business's ourselves. Does anyone have a rough idea of the rates to charge, note we are not London, Sunderland to be precise. Do you think also that if we advertised Morrisons for arguments sake, would Tesco kick up a stink when we pull in there or should we use that as a bit of leverage to get a better price from whoever.