Where can I find a car rental place that does direct billing to insurance co but no deposit?

Enterprise car rental will want a valid credit card authorization for a "deposit".

There really is no way around the "deposit". However, if you are under 25, have a parent rent the car from Enterprise and then you drive your parents car.
Not possible.

They're ALL going to want a credit card as security deposit, in case you run off with the car, or damage it.

I'm a fleet manager for a Toyota Rent A Car agency in MD and I have NEVER heard of that! I rent to people whose ins. companies pay the bill all the time and never needed a "deposit". What we do is gather all insurance info, and I call the ins. co. directly and verify direct billing with them, get claim #'s and account numbers, and I don't need anything from the rental customer at all. Check with your nearest Toyota Rent A Car company at www.toyota.com/rental, and see what they require.