1. A system for identifying sources to provide items, the system comprising: one or more data stores; and one or more computing devices in communication with the data stores, the one or more computing devices configured to: receive a request to identify a potential source for an item that is offered by a number of sources on a network-based service, wherein the number of sources is less than a threshold, and wherein the threshold is determined at least in part by analyzing statistical information relating to searches performed by a plurality of users of the network-based service; obtain one or more keywords pertaining to the item from the data store; obtain search results in response to submission of the received keywords for the item to a network-based information resource, the search results comprising one or more sources for the item; generate a source score for each of the sources returned by the search results; identify a source from the returned search results as the potential source if its source score satisfies a selected numerical criterion; obtain contact information for the potential source; transmit an offer inviting the potential source to provide the item on the network-based service using the obtained contact information; and add an entry to at least one of the data stores after the transmitted offer is accepted, the entry indicating that the potential source provides the item on the network-based service.