It's not as tart as other kinds of sparkling megaswill cider, so use mainly dessert apples for it. Correct me if I'm wrong, but my assumption is that your girlfriend is not a cider connoisseur, therefore would probably not notice if the apple blend wasn't quite right. I'm sure she will be very pleased with any fairly light bodied, inoffensive, clean, crisp sparkling cider that her significant other makes her. Cloning isn't as important a concept in the cider world as it is to beer.
I would use a blend of the eating/dessert/sweet apples available at your supermarket, ferment to dryness, add a couple of tbsp of artificial sweetener to your bottling bucket along with the required amount of bottling sugar for a sparkling cider, then bottle.
If you want to use storebought apple juice, go right ahead. I personally would blend up the apples in a food processor until they're in little chunks about the size of a fingernail, fill up the fermentor with them, put some pectic enzyme and 1/3 a tsp of acid blend (and some campdens if you want) into a bottle of storebought apple juice and pour it on top, pour more juice on top until the apple chunks are submerged, swish/stir it around a bit, leave it for 24 hours, then pitch your yeast. That should work nicely.
But it's Strongbow, so just fermented storebought juice should probably be fine.