You can find news coverage of previous democratic primaries using the databases and links in the "media sources" boxes on this guide's news, polls, and candidate background page. Try using search terms like "primary" or "nomination," "election" or "campaign," along with the names of individual states if you like. The databases might be particularly useful because they have publication date filters that will allow you to quickly narrow down on articles published in 2008, 2012, or 2016.