Hi Kate,
I don't know of any American Insurance Companies that will offer short term coverage to non-residents. Perhaps someone else here will have some knowledge about that.
I do know that all Car Rental companies will try to sell you insurance on your rental for an additional fee. This is always done when you pick up your rental. You can view Avis' page about U.S. Coverage Policies here to get an idea of what the agencies offer. If you do buy coverage from the rental company, be sure to find out exactly what you are covered for.
http://www.avis.com/AvisWeb/JSP/global/en/rentersguide/policies/us/coverage_policies.jsp
I have never actually purchased coverage from a car rental company because my credit cards always offered coverage on rentals paid for with that card. I suggest you check with your credit card companies to see if they offer this and what the coverage is. It's possible that the coverage will be enough for your needs and you can waive the additional coverage from the rental agency.
Good luck!
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(¸.Michelle ¸.·
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