You should have no problem finding one, they are almost always close to the port, ask anyone once you are there and they can direct you. I remember one in St. Thomas right by the ships on the pier, can't miss it.
My tip of the day. At the main location in Charlotte Amoile of Little Switzerland there is a FREE internet cafe located upstairs (stairway in the back of the store on the right side). It is sponsored by Baume and Mercier . If crowded they limit you to 10 minutes or so, if empty no one will bother you. The space is airconditioned (nice and unusual for downtown) and there is a snack bar /coffee shop and clean /empty restroom.