Ahm that - as you said - is a smock dress, not a dress with smocking. I can't see anything quite like that on eBay at the moment. Have you tried Laura Ashley shops?
I wonder if you can still get the knickers. I remember that a lot of them, especially for the younger girls were actually plastic frilly knickers which covered the big terry nappies. There isn't much call for them these days as virtually everyone uses disposable nappies.