Try
www.travelguard.com
Travel Guard is the company that offers insursance on owners'
DVC stays, so they are very familiar with DVC, points, and all the associated issues. They're also probably the largest insurer in the
trip insurance market. [ETA: I have also checked with Travel Guard specifically about covering rentals between two individuals, and they have no problem at all with that. They do it all the time in timeshare/vacation home rentals.]
We have used Travel Guard a number of times for non-DVC trips. We've had to file two claims over the years and they were great to work with.
As noted above, be sure you understand what the insurance will pay for and what it won't. They will generally pay for most non-refundable expenses if a vacation is canceled because of one of the covered risks. Those risks include medical problems for the traveler or immediate family, and many other issues.
Most trip insurance companies have deadlines by which you must have purchased the insurance -- usually within 2 weeks of booking the vacation. So you can't book, find out later you've got a problem, and then get the insurance.