Renting a car is the only option, I doubt you would want to buy the car that you would sell a month after you bought it.

As for the companies and the deals they offer, from my experience I would recommend Budget. They had and still have, by far, the best deals available. For example, on my last trip (in 2007; NYC - SFO) I rented a Ford Taurus '05, monthly fee, unlimited mileage (I put more than 5000 miles on it), Loss damage waiver and Supplemental liability insurance included. It cost me just over $1000, and they charged me no drop-off fee.

I recommend you do your homework and do some internet research. For example, on this last trip my starting point was NYC, however, rates are higher in big cities and at the airports. I seched for Budget locations in NJ, and found the best deal in a small, remote location some 1,5 hrs from NYC. I asked a friend to give me a ride, and saved more than $1000 by just renting a car outside a big city/airport.

Good luck in your search and be patient!