I like these sources:
http://magazine.uchicago.edu/http://harvardmagazine.com/http://sciam.com/Scientific American can get a bit too casual for the GMAT, but it's worth including if you aren't comfortable with science topics.
If you have a friend who's good at reading comprehension (in general, doesn't have to be for the GMAT), you could ask him/her to read some of the same articles and then you give an oral summary to your friend. See whether you get the main points. If your friend picks up on things that you don't, or you misinterpret something, have your friend show you how s/he read and interpreted that text.