Technology journalist (Forbes, Mashable, The Wall Street Journal, The Huffington Post), digital marketer (The New York Times, Contently), startup publicist (HaulHub, WeHostels), entrepreneur (Deliverish), and MIT Sloan MBA 2015.
Seek out "contributors," "guest writers," or "freelancers" who write for the publications you've identified as relevant. These writers are often freelancers with a little extra time to write. Your offer must be compelling, though, because writers tend to enjoy writing for large publications, not startup blogs. That is, unless the topic is a huge interest of theirs in the first place and the pay is good.
If you have a budget, you could also consider checking out Contently, a network for highly talented freelance writers. They serve large brands for the most part, though, and are often over-priced for startups.