There are a number of blog writing / content creation & management resources: Zery's, ion, Ceros, Kapost, for example.
However, your best bet is to handle this process in-house on behalf of your clients. Use LinkedIn to connect with freelance writers. Your best results will come when you can partner with a freelance writer who has a basic understanding of the topics you are looking to have covered.
Let me know if you need any more guidance. I've operated an agency for 15+ years and now consult on the topic of running an agency (specifically outsourcing such tasks).
Think of your business goals and how content can help these goals before you plan to hire *content writers*. Most often, organizations who need an architect for their building's positioning, end up hiring brick-layers.
To me, there is no role as such of *content writers*. You should look people who can understand your business goals, plan content audit and gap analysis for WHY the blog may help your business, plan a voice and tone for your brand, and then work on a calendar. They should be able to measure the effectiveness of content too.
Mere content writers rarely (very rarely) bring any value to the table. You need content strategist who can work with marketers and product team to ensure that content is relevant, meaningful, and is aligned to your goals.