- Khan Academy (includes videos and practice passages)
- Jack Westin (one CARS question a day)
- Princeton Review (one short passage)
- Kaplan (one short passage)
There are remarkably few free CARS practice passages available on the Internet. If you believe, based on diagnostic testing or prior struggles with reading comprehension, that CARS will be a difficult section for you, investing in additional practice might be a wise investment.
While there are workbooks you can purchase from NextStep, Exam Krackers, and others, I am wary about using these resources to practice, as computer-based testing carries several nuances that one must specifically prepare for. In essence, you should practice like you play.
Some of the only CARS computer-based practice you can purchase, absent that which is included as part of a larger prep course, is the NextStep MCAT QBank, which ranges in price from $50 to $99.
See the Strategy page for information on how to tackle CARS.