I used to work as a 'vehicle delivery agent' (more commonly known as a 'trade plater'), delivering vehicles all over the country. I could be driving a Porsche in the morning and a BMW 7.5 tonne truck by the evening so, obviously, I had to have just the sort of insurance that you're talking about. I can't tell you how much it cost because I worked through an agency which had a group policy for 250 drivers. However, some of my colleagues broke away from the agency to work independently, so I know that individuals can get such insurance as well. As I've said, I've no figures available but I know that the agency paid a fortune for their policy (and they had to agree to pay the first �1000 of any claim).
Also, any respectable garage will have similar cover for when they 'road test' vehicles after they've completed repairs.
Hoping this helps,
Chris