This paper presents OptiSource, a novel approach of source selection that reduces the number of data sources accessed during query evaluation in large scale distributed data contexts. These contexts are typical of large scale Virtual Organizations (VO) where autonomous organizations share data about a group of domain concepts (e.g. patient, gene). The instances of such concepts are constructed from nondisjointed fragments provided by several local data sources. Such sources overlap in a non mastered way making data location uncertain. This fact, in addition to the absence of reliable statistics on source contents and the large number of sources, make current proposals unsuitable in terms of response quality and/or response time. OptiSource optimizes source selection by taking advantage of organizational aspects of VOs to predict the benefit of using a source. It uses an optimization model to distinguish the sets of sources that maximize benefits and minimize the number of sources to contact to while satisfying resource constraints. The precision and recall of source selection is highly improved as demonstrated by the tests performed with the OptiSource prototype.