Kikkoman soy sauce
Although you can use any brand of soy sauce for a West Indian curry, if you haven't yet tried this brand of Japanese soy sauce, I urge you to do so! It is a taste revelation.
The cost of a 1 litre bottle is a little eye-watering, but smaller bottles are available (though they're not such good value). A litre will last you a good long while, and only improves in texture and taste as it ages. Store it in the food cupboard.
Supermarkets will probably stock this in the World Foods aisle.