This paper presents OpenXView, a model for open, large scale XML data integration systems, characterized by the autonomy of users that publish XML data on a common topic. Autonomy implies frequent and unpredictable changes to data and a high degree of structure heterogeneity. OpenXView provides an original integration schema, based on an hybrid ontology - XML schema structure model. We propose solutions for several important problems in such systems: easy access to data through a simple query language over the common schema, simple data integration view management when data changes and scalable query rewriting algorithms. This paper focuses on source identification for query rewriting in OpenXView, i.e. the computation of combinations of sources that can answer a user query. It proposes two algorithms for minimal source combinations, scalable with the number of sources. The first one is based on a general branch-and-bound strategy, while the second one, very efficient, is limited to queries whose number of attributes is no more than 8, sufficient in most applications.