As I like to buy T-shirts that have some relation to the islands I visit, I have bought good quality ones at Guavaberry and the Aedes Aegypti mosquito shirt from the lady at the Marigot market. She's in the back near the spice man and the rum dude.

Apparently, every one on the island makes their own "proprietary" blend of magic spice, so you can buy it from virtually any stall that sells spices.