Service Grids allow providing all kinds of resources ranging from simple storage space to complex functionality. Provision of data, however, is usually limited to connecting data sources to the Grid via service- or document-oriented interfaces. This approach hardly implements the idea of virtualized, easy-to-use resources. We propose a virtual data source that provides structured data according to user demands instead of forcing users to adapt to existing data sources. To this end, existing data sources are selected by their suitability for a given query. Their individual results are then dynamically integrated into the overall result presented to the user. In this paper, we present the background, the main concepts and the architecture of our virtual data source.