Training Dataset required for Classifier
You have at least three options:


Use some of many available datasets (for example: BBC documents); if you need more, simply go to Google Scholar or any similar service and search for classification news politics sports. In the articles you may find many references to available datasets;
Crawl any news service, and use clustering techniques to group articles into clusters (this often separates the articles along their domain, such as politics, sports etc.), and label the articles by their affiliation to a cluster;
Crawl politics-specific, sports-specific news services and use them as labeled datasets.