The reason it's difficult is because there is almost no way to know/get the exact apple combonation used in each manufacturer's juice. and it's especally hard for a homebrewer to make much of the north-american commercial ciders, as they are sparkling and sweet. The Woodchuck "Granny Smith" (which is my least favorite) would probably be the one you'd be able to clone the easiest, as it says it's made from just granny smith apples. I'd think the way to duplicate this recepie would be to get a bunch of granny smith juice, Ferment it out using a nutrial yeast (notty, or 1118, or champaigne), stabalize it, water it down to about 5% abv using an apple juice and water mixture (not sure on the amounts), and force carb it. this would probably get you pretty close, but I don't know why you'd want to duplicate anything but the woodchuck 808 dark and dry (the only one I can stand by them).