This subject was discussed in detail in the March / April time frame on this server.  Try doing a search.  The jest of all of it is there appears to be 3 reputable providers.  Each seems to provide a good service.  Check them all out to see which best suits you needs.  I personally used Divers Security Insurance and they paid off just fine when needed.  As many on this list will attest if the NPS does the rescue most likely you won't be billed.

http://www.diverssecurity.com/
http://www.diversalertnetwork.org/
http://www.travelex-insurance.com/Static/Welcome.aspx

AAA plus covers medivacs also May also be included in Blue Cross coverage, pg 32

Check all the fine print on these policies. I believe on Diver's you have to arrange the evac through them to be reimbursed. Given that your calling NPS first (rather than waiting for an okay from Diver's), it may invalidate the reason you're getting extra insurance.

This came up recently and I checked my own health insurance - Blue Shield. It covers air ambulance so an evac is covered. It is subject to the usual deductions, etc.

Hope this helps.
Good boating.