The official source is the most recent Form 13F that Berkshire Hathaway, which is filed with the Securities & Exchange Commission on a quarterly basis . You can find it through the SEC filing search engine, using BRKA as the ticker symbol. and then looking for the filings marked 13-FR or 13-FR/A (the "/A" indicates an amended filing).
As you can see by looking at the 13-F filed for the quarter ending September 30 , the document isn't pretty or necessarily easy to read, hence the popularity of sites such as those that Chad linked to. It is, though, the truly official source from which websites tracking the Berkshire Hathaway portfolio derive their information.