HARO's great when a journalist posts a request to it, but the limitation is that only a very small portion of journalists post for sources before writing their story. But almost all journalists are tweeting about and writing about what they're interested in, which is why we built Muck Rack (I'm the founder, so a little bias obviously). We index all the tweet and articles of journalists, let you search them to build a media list, and then enable you to pitch them through our messaging system or by email.
We also have an email alert tool that will notify you whenever a journalist tweets about or shares a story with your keyword.