Article Forge uses Artificial Intelligence to create unique, readable articles for your SEO needs. It can also add images, videos and links to the articles. You can setup automated posting to your blogs with their scheduler.
Private Label content, known as PLR (Private Label Rights) is content shared by many purchasers. If you want it unique, it'll take a bit of rewriting. The biggest PLR search engine, Big Content Search, hosts over 128,000 articles and 1000 ebooks which you can use any way you like.
For those high-quality blogs you'll want to get quality unique content. There are many services out there but, being biased of course, we only use our friend Colin Klinkert's service Green Light Articles.
PBN.bz offer three different levels of content: from SEO content, lead gen/affiliate/AdSense site content to well researched articles on a topic. All content is manually written and unique.
With all the above sources it's highly recommended you use an article spinner which will make your articles unique. Just make sure to run your articles through grammar check so you don't get hit with a penalty. By far the best article spinner is Spin Rewriter, developed by our friend Aaron Sustar.
Wayback Machine Downloader searches Archive.org for forgotten high-quality content that you can then use for any website.