I have a trip report on here somewhere from June of this year.

Since it's still considered off season, if you're looking for specific items, I would suggest ordering ahead from Gourmet Foods not far from the base.
Gourmet Foods

There are two good groceries nearby for most other items, we had no trouble finding most things in June.

We went to Gonzales in Kingtown for booze, great selection - They take orders if you don't want to go there.

Gonzales

Provisioning for other items along the way you will not get the same selection, but veggies can be found on most islands, Union had a good selection and is at a good half way point of you're returning back to St. Vincent.

Bread, ice and other treats can be purchased from Boat vendors in most anchorages.

Have a great trip and I hope you'll do a trip report!