If you are renting a car american express has insurance that covers all the north american extra insurances that you need.
Additionally a platinum charge card comes with nice things like hilton gold card and free airport lounge access as well as the insurance which make the £300 fee good value if you are a frequent traveller (but obviously not so good value if you are not !)
Otherwise check out moneysupermarket.com which has a nice search function and comparison page.