I believe third party liability insurance is required for car rental in Cancun. What is the best way to get this insurance and what can you expect to pay? Thank you

You must get this insurance from the rental car company at the time of rental. Its hard to say what it will cost. That depends on the company, the car, the amount of coverage, the deductable. As a general rule, expect it to double the price quoted for the car.

If you want get an idea, go to the Easyway site and start to make a reservation for a week. All the insurance options for the car model/car size you chose will be displayed with daily fees (according to the model/size) for each type of insurance. When you check boxes for what insurance you want, you will see the whole quote for rental and insurance and taxes for the week.
(With Easyway, what you see online is what you pay on arrival. No flim-flam.)

Note: We've recently paid about U$40/day for a week's rental, taxes and full insurance (not just third party liability) for a car seating 4-5, with closed trunk to store/hide luggage. With Easyway.

I've also used Easyway the last few trips and once used America, both were fine and will quote all the insurance. Also consider getting the zero deductible insurance (EPC). A small sedan should cost about $35-40 per day, a bit more if you want an automatic.

Thank you all for your very helpful advice. Looking forward to being in the Yucatan next week. Samba Chula, I have been following your discussion in the Brazil forum. Driving from Salvador to Mucuge and back recently was a special experience.

Do make sure to ask or read what kind and amount of coverage you are buying. There can be a huge difference in the deductable amounts.

Enjoy!

@hoyas1-- Hope you enjoyed your visit to the Chapada Diamantina!

[Define "special" experience. Could be taken for a Chinese curse, like "May you live in interesting times.". ]