There are two large supermarkets in Rodney Bay and one between Rodney Bay and the marina, you'll pass them on the way from the airport to the marina, just ask you taxi driver to point them out.
You can grab a bus outside the marina for 1.50 EC and then take a taxi back or dingy to the far end of the lagoon, you'll see a small dingy dock. Take the walkway to the main road and turn left, the supermarkets are right there.
If you arrive a day early, the market in Castries has loads of fresh fruit and veg plus local spices and sauces.
When is your charter?

Drink all day at home, your friends worry about you; do it on vacation and they say "what a good time you're having". Save your friends needless worry, travel more!