Are there doctors in the New York City region that specialize in "larger" or "extra large" breast implants? (500cc and above)

I have upsized many patient to larger sizes. It is usually not a problem, I don't like the saline as the larger sizes look too round. I do prefer the nicer newer silicone ones.

If a patient wants a larger implant and their body can handle it, it should be OK. I do not think you have to travel to Vegas for this.

Sorry that I cannot provide you with specific names. I would suggest starting with the American Society of Plastic Surgery and/or the Aesthetic Society of Plastic Surgery to obtain a list of well experienced board-certified plastic surgeons.
Then, I would suggest you visit a few surgeons whose practices concentrate on aesthetic surgery. Ask to see lots of examples of their work and preferably speak/see patients who have had similar procedures done.
Be very careful with your decision-making. You will find, while doing your due diligence, that there are many different “specialties” who will offer their services to you; again, I strongly recommend you concentrate on surgeons certified by the American Board of Plastic Surgery.  You may find the attached link, dedicated to larger breast augmentation surgery outcomes, helpful to you as you learn more.
  Best wishes.