I recently travelled to the western U.S.A. and purchased a travel insurance policy from Access America Travel Insurance.
Their insurance policies are sold thru Alaska Airlines when you book your airline tickets. A harsh warning; this company is wracked with completely unethical and most likely, illegal practices. Their offices are staffed by semi-mentally retarded people who can barely speak any English.
They rarely will pay out on their medical or emergency medical claims.
A word of advice, shop around for your insurance. It is important to have good, reliable medical insurance coverage when travelling to the U.S.