How might one find a good tech startup writer for a few articles per week?

Find someone on Elance.com for a fast hire. LinkedIn.com for a more targeted approach at someone looking for a full time job... but expect a standard recruiting/hiring cycle time-wise. TextBroker.com will get you through in the meantime with pay per word writers on demand.

Scan the comments section.

You've already identified venues where the in-house writers have the experience and skill set of interest. Well, in many cases you'll find amateurs and knowledgeable experts who already care enough about those subjects to voluntarily troll the internet looking to post their viewpoint below the official article.

Find someone whose tone is collegial rather than adversarial, who seems to know his stuff, and whose natural written style requires little babysitting. Hire that person!

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.

One of the best places to look for writers is Problogger. It costs you $50 to post a job but you will find one of the best talents over there.

You can hire great freelancers on oDesk and Elance but you need to know how to search for them. They will cost you less for sure. Look for people who have written in your specific niche and ask them to write you a post or 2 to see what they are capable of.

Right now I'm teaching people especially about how to find, hire & manage freelancers. I have hired more than 100 freelancers so far so If you want to learn more about how to do it, feel free to schedule a call.

Elance.
Don't stop taking massive action.
Best of Luck,
Michael T. Irvin
michaelirvin.net
My books are available exclusively through Amazon Books. Check out my book "Copywriting Blackbook of Secrets"
Copywriting, Startups, Internet Entrepreneur, Online Marketing, Making Money

Odesk/Elance is good.. As a web content writer myself, I usually get found by clients as they invite me to apply to their job (usually related to, for instance, my forte: health/nursing/psychology/internet marketing, etc.). From there, the client selects the best bidder (from the proposed rate, experience, and even some specific tests to determine if we are really a good fit). Hope this helps!