Credit cards are pretty much the cheapest way to finance a car just now. But you'll be doing well to get the limit to cover a new car, or even many decent second hand motors.
However, if you can get the limit on a card, assuming the dealer is happy to take payment via a card (they may ask you to pay the 3% fee for credit cards), it will likely work out the cheapest, other than saving up your money and buying outright.
You buy it on a 0% purchases card, and make monthly payments to the amount you would with a loan or whatever, then when the 0% runs out, transfer to a 0% balance transfer card (typically a 2.5-3% charge here). Then arrange the same payment schedule.
So say you want to buy a £20k car, paid up over 5 years. This would cost you £364.21 per month according to compare the market, with an interest rate of 3.6%, and you would pay £1,852.60 in interest over the period.
Again according to compare the market, the best purchases credit card is from the Post Office at 25 months interest free. So buy it on that, and pay £364.21 per month for the 25 months. After 25 months you are down to £11,258.96. Transfer that to the best balance transfer card, which is currently the 36 month Barclaycard with a 2.39% fee, and your new balance is £11,528.05, so a £269.09 charge.
Then continue making payments as before, and the final payment will be made on month 32 having only paid £269.09 in "interest" rather than £1,852.60, a saving of £1,583.51 over what is likely to be the next best way to finance the car. Of course, if the dealer asks you to pay the fee for the £20k car, that comes in at £600, which reduces the savings, but you'd still be almost a grand better off.