is perfectly OK as long as they have NO added vitamin C (smaller doses of natural ocurring vitC is OK, and that is usually listed on the part of the label which lists the "daily recommended dose, that this juice will provide"... I look for those with 0% or maybe up to 5%)
Most people use apple juice, I personally got too bored with that taste, and found some of the R W Knudsen brands to fit the bill perfectly, their Apricot Nectar, Peach Nectar or Pineapple/Coconut are all OK...
I am sure there are other choices, just always check first the ingredients for absence of ascorbic acid and then the "recommended dose" part of the label.
If you eat any fruits with naturally occurring vitC, it is fine, and does not interfere with the process.
Any ascorbic acid product are best taken at least two or more hours away from the Miracle-Mineral-Supplement .