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This one? It was for Pokedex 3D. As of now you can't get the original, and I think the Pro version already has the Unova guys in there already.
Serebii has image of the rest of the Pokedex, which you can scan in/use for shit.
Any other QR codes you may have seen are probably for Animal Crossing New Leaf, which has a QR Machine available after a certain point in the game which allows you to scan in designs for clothes or paths or whatever.