We are U.S. expats visiting the U.S. for a few months. What is the most reasonable way to obtain car rental insurance other than buying the insurance through the car rental company?
Depending on where you currently live, an option is car hire excess insurance. I have one of these policies and for an annual fee it covers any number of rentals throughout the year. It covers the amount that you would be liable to presuming you take the minimal insurance offered. I had a hire car stolen which would have cost me about 2000 Euro and the car hire excess company paid it off directly to the rental company in days. The annual policy cost less than 100 Euro. I probably rent for 100 days a year.
Rather than giving you a direct link to a sample company, I suggest you do a web search for car hire excess insurance in the country you live.
One possible way is to use a credit card that offers their own insurance for car rentals. Of course this is only possible if you actually have such a credit card, but it's worth checking your cardholder agreement to see whether yours does.
If you are covered this way, it is usually automatic when you use the credit card to pay for the rental, and no other steps are necessary.