I struggle with science passages. What sources do you recommend for practice?

Good question!

First, we have a few blog posts that talk about how and which types of articles you should be reading for the GMAT:

1. GMAT Reading List

2. GMAT Reading — The New York Times

3. Reading for the GMAT — The Economist

For science articles specifically, I recommend reading from the following sources:

1. Physical + Biological Sciences: Scientific American

2. The science and tech section of The New Yorker

3. The science section of The New York Times