I've just discovered Newsela (thanks Glenn Wiebe!) Newsela is a site that offers nonfiction text (including news articles and primary sources) at multiple reading levels.
Take for example this February 28, 2014, Los Angeles Times article, "Oldest skeletal remains in Americas to be reburied."
The first paragraph of the original reads: "The skeletal remains of an infant who lived in what is now Montana about 12,600 years ago will be reburied in a formal ceremony now that scientists have sequenced its genome, researchers say."
Newsela staff rewrote the article for several grade levels.
Their fourth grade version (690L) is titled: "Ancient Native American boy's bones to be reburied." The first paragraph reads: "Sometime soon, pieces of a very important skeleton will be reburied. The bones are the remains of a young boy. Scientists believe he lived in what is now Montana about 12,600 years ago."
At the sixth grade leve (1000L) it reads: "At the request of several Native American tribes, an ancient human skeleton will soon be reburied in a formal ceremony. The bones are the remains of an infant who lived in what is now Montana about 12,600 years ago. Scientists have already completed a sequencing of the boy's genome."
Newsela is better for geography, world and U.S. history, and current events than for Montana history, but there are a numbers articles about Montana, collected into their text set, "Montana News."
Newsela also offers simplified versions of some primary sources through its Library, including this interesting (and extremely patronizing) article on Jeannette Rankin's election to Congress in 1916.
To access Newsela resources, you'll need to register--but registration is free.