Vehicular ad hoc networks (VANETs) are viewed as an effective way to disseminate content among vehicles on the road. While most of the current research efforts in VANETs focus on improving the packet delivery performance, only limited work aims to provide cost-efficient solutions for content downloading. This paper proposes a novel approach for the vehicles to download a common content in a cost-efficient way. The basic idea is to stimulate the vehicles to download the content cooperatively in mutually disjoint coalitions. To study the cooperation among the vehicles, a coalitional game model is proposed. Moreover, a distributed coalition formation algorithm is designed to obtain a stable coalition structure and minimize the total communication cost. Numerical results show that the proposed content download approach can achieve a considerable communication cost reduction compared with the non-cooperative case.