Organic Grape in a jar, or frozen concentrate apple so far. Haven't tried pineapple as it has a little more vitamin C. I bought them at Whole Foods but I don't remember the exact brand. Just get one with the lowest vitamin C Daily allowance listed on the label.
Lately I used some Cranberry Concentrate 100% juice because I was out of the others. Seems to work very well, Miracle-Mineral-Supplement still very strong. The brand is "Vitamin Shoppe". You just mix in 8 teaspoons in 8 ounces of water. I just put in 2 tablespoons. Then I use 4 ounces with the Miracle-Mineral-Supplement .