Most fruit juice has malic acid in it. How much compared to apple cider, I don't know. You'd have to do some research on that. I've never used anything but the apple cider. That's for softening up the stones.
You can get some malic acid from http://www.bulkfoods.com
Anthing more than a scant teaspoonfull in a quart of distilled water is quite tart. I'd go with the apple cider. The stones need to be softened up or they will be hard on your bile duct coming out. It's your choice.