I often rent cars and end up paying insurance. My credit card covers collision damage waiver (CDW) but not liability, which is required by law in states like California. Is there a policy I can get from an auto insurance company that will give me liability for a good price? I'm tired of paying $10/day for something I never end up using.

Yes, you can. A friend of mine has does not own a car but wanted rental car (traditional + Citycarshare) coverage. He was able to get it through USAA (military families), but my guess is that you would be able to get it from other companies as well. It was something like 100 bucks for the whole year.