Unfortunately unpasteurized apple juice has become incredibly hard to find. I used to be able to get it regularly at a large specialty health food store in Chicago that carried a lot of items you couldn't easily find. Now they only carry it very occasionally....it's in the freezer section and the quart or half-gallon bottles are frozen solid. Sorry, but I don't recall the brand name.
Of course, a more dependable option would be to juice your own apples.
When I couldn't find the apple juice, I just used orthophosphoric acid and got good results. It, too, is a bit hard to track down. I know of two...one is Ortho-Phos (and I can't for the life of me remember that brand either!) and the other is PhosFood by Standard Process...phew! at least I got one of them!
Sorry for the lapses in memory....it has been a long, stressful week and I'm perimenopausal...LOL!