Where do I find extensive taxonomy repositories?
Take a look at Open Calais (http://www.opencalais.com/) They have an extensive tagging system that is free to use. They even have a Drupal module you can use: OpenCalais. Details from its project page:


  The Calais Collection is an integration of the Thomson Reuters' Calais web service into the Drupal platform. The Calais Web Service automatically creates rich semantic metadata for the content you submit – in well under a second. Using natural language processing, machine learning and other methods, Calais analyzes your document and finds the entities within it. But, Calais goes well beyond classic entity identification and returns the facts and events hidden within your text as well. The web service is free for commercial and non-commercial use. It requires registration to obtain an API Key.


You can use such a service to find/suggest tags for your corpus of content.