There are any number of places in St Thomas to make this kind of purchase . The most well known would be Royal Caribbean (nothing to do with the cruise line).
You should do some comparison shopping at home or on line before you shop in St Thomas.
I'm travelling to St. Thomas on Carnival in March, and while the temptation is great, just a few words of wisdon from a photo instructor....!
I do believe in supporting local (independent) stores. Certainly get a price of exactly what you want before you go. My caveat is that while the price may be 'too good to be true' the warranty may not be worth the paper it's printed on should you have a problem when you get home. Do check with your local dealer for what IS and ISN'T valid on an international warranty!
I feel that a camera has to have a heck of a price (before tax) at least 20% below what I pay at home for me to really consider that. Well known brands (Sharp, Canon, Nikon, etc) always back up their warranty, but check before you leave home.
Ships run specials from time to time on cameras that are really good prices--again check with what they run back home, and do some comparison shopping.
Good luck, and I hope this helps a little.