Hi Ken, the subject of car insurance is oft discussed on this board! Try using the search facility to see previous discussions. As I see it there are four alternatives for your car insurance (and SLI is essential in my opinion, particularly in the litigous USA):
1. Buy the insurance through your tour operator.
2. Buy the insurance from the car hire company when you collect the car.
3. Buy your insurance from an independent insurance company.
4. Get your insurance policy as part of your travel insurance.
In my opinion the prices charged in 1 and 2 are complete ripoffs, and I have never used either of these options.
For option 3 there are a couple of companies which offer independent insurance, Natwest being one (but only if you have one of their [gold?] credit cards). Other people should be able to give you details. The prices I've seen quoted are much cheaper than 1 or 2.
I normally take out option 4. As far as I know, only American Express include car hire insurance as part of their travel insurance policy. For me this has worked out much cheaper than the other options. Please ask if you want more details.
Regards
Rob