Forming dynamic supply chains within a Virtual Enterprise relies on efficient search and negotiation of web services from external suppliers at the point of need, perhaps using negotiation mechanisms provided within an electronic marketplace. This paper proposes the use of cooperative service requester agents within the "alternative offers" protocol. Under this idea, service requester agents approaching their pre-set negotiation deadline yet having failed to secure even a single offer would issue a call for help, to which other requester agents will respond by donating their superfluous offers. In this paper we propose a formal model of this protocol and investigate its effect on improving the success rates in procuring web services.