Newsela has been an AWESOME resource over the years. It was free, but has moved to a paid subscription model. (I know, business is business.) ReadWorks has also been a generous location to find texts, but I found that the writing style of the articles was simple and not rich.
I strongly believe that "rigorous texts" doesn't mean "harder texts". Rigorous texts support deeper level thinking. Pulling from a variety of sources helps that happen.
Here are 5 sites that I found where there are an abundance of resources by an abundant amount of authors. Happy reading!
You will find leveled non-fiction texts at Tween Tribune. You can create a class and assign articles.
2. Wonderopolis FREE
I've used Wonderopolis in my classroom as a search engine for my kiddos during research projects, but I hadn't thought of using it to find articles for topics we are all studying together.
3. Reading Vine FREE
At the Reading Vine, you can choose articles and books and download them as PDFs. There are questions attached that you can use as well. There are primary sources available here as well.
4. CommonLit FREE