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.