After spending a couple of hours on this, I've found that the following sources are NOT as useful as one might suppose:

* NYTimes, the Economist, BBC - paragraphs are often too short, too easily digestible to be useful.
* New Yorker - many paragraphs lack a main point, because articles often feature chunks of meandering first-person narration
* New York Review of Books - some articles are very GMAT-like, but others are overly polemical on political issues and others have an overly informal tone (e.g. "Still, like all notable English novelists, he was a tricky bugger.")

The best sources I could find are a couple of alumni magazines and Scientific American. These describe scholarly research in relatively formal prose, with the aim of making the research intelligible to educated non-specialists:

* https://magazine.uchicago.edu/ - particularly articles in the "Investigations" tab
* https://harvardmagazine.com/
* https://sciam.com/ (This can get a bit too casual for the GMAT, but it's probably worth including because so many of our students get freaked out by science passages on the GMAT.)