Linked Open Vocabularies (LOV) is great initiative when you are looking for existing ontologies that describe certain concept. You could simply search for a certain class or property and get links to the matched ontologies as a result .
The links usually contain descriptions of the classes and properties of an ontology as well as file in OWL, RDF or other Semantic Web/LOD standard format of the ontology that you could easily convert to OWL (if necessary) using online converters for instance.