You can find the color change shirts in St. Martin at a store called Del Sol. You can get a map to find this shop (and others) on the ship at shore excursions desk. We took the water taxi and then walked to the shop. They have a big variety of shirts and other items. I think the t-shirts were $18-$20 dollars, $24 for long sleeves and around $30 for a sweatshirt. Kids sizes were a little less. Have a fun cruise!