If I were you I would do a few trial runs on smaller articles/projects with some people that you might find in the marketplace here. Test them out, and see whose writing style works, and find out who is a best fit for you, in terms of a professional relationship. Theres also the other freelancer sites out there like getafreelancer.com guru.com etc. I couldn't recommend anyone personally though.
Usually people who specialise in SEO/LSI articles can do newsletters. Finding the right price may be difficult, however -- newsletters usually require more work than the average article, so it will naturally cost more. When I used to do it I charged $50 per newsletter, and that was low. I would suggest finding a freelancer who isn't quite established yet. There are plenty of freelancers in the buy/sell area, if I recall correctly. The person does not necessarily have to specialise in SEO, however, just web-friendly content. Make sure they have samples, though.
You're in a right track asking that here in DP. Im sure lots of companies and freelancers would love to do the project with you. Best thing is to post a thread asking for a newsletter writers and include all the details that you need. Good luck in searching!
Consider typing one out first, and then posting it here for critique. Once you get the first one down pat, the rest of them will be, or should be, easier. You may not even need a, "writer."
You should try the content section here, though you should be specific in your demands as there is a wide variety of writers who may apply for such a job. Most of them won't be great, but they will be cheap. If you want to get a good writer then it will pay to do some digging around and checking of samples etc. Sometimes it is worth paying a little extra for someone who can do a good job.