From Semantic Web Standards
The RDF/OWL files for the SKOS and SKOS-XL ontologies can be found in the RDF vocabularies section of the W3C SKOS web site.
The RDF files for SKOS are also accessible using content negociation from their respective namespaces, for instance http://www.w3.org/2004/02/skos/core#. Please note that a normal HTML browser will get an HTML page when accessing these namespaces!
Please note that the W3C SKOS web site also maintains the history of the various RDF filed published before the 2009 Recommendation version.