The retailer would be the best person to ask this question of, so I would give them a call. I suspect the answer will be no, though you should check cause you never know.
I am not sure what you mean by 'designer' but my understanding is that Cartier, Tiffany, HW, etc will only sell completed rings and not settings (I assume the same is true of Bvlgari but you could always check). There are setting companies that some would consider 'designer' such as Tacori, Vatche, Ritani, Verragio, etc that sell settings.