If you buy her the alcohol, you have now become part of the problem. Don't think at some point it won't get thrown back in your face. How can you be mad at me when YOU bought the booze??? And there is NO guarantee that she won't still go to the store for more. You can disable the car, the A will find a way.
We have to learn to surrender, that we are powerless over this disease. As long as you find ways to cushion the blow, it will certainly continue. It is her right to decide her own path, and if she chooses to drink there is nothing you can do about it. What you CAN do is take care of yourself. Find AlAnon, get help for yourself.
I know you're in a tough position. When my RABF was actively drinking, I was worried sick when he would leave the house in the car. But I had to leave him to his Higher Power. He did eventually rear end someone, and it was the bottom that finally got him to seek treatment. Had I interfered, he might still be drinking or even dead today.