There is a store in the old Purple Turtle spot. I think it's reasonable stocked for what you're looking for but I haven't seen it for myself.

One thought for you. Before I moved to Tortola, I was spending 3 or 4 trips a year on Anegada for 2+ weeks at a time. We always provisioned thru Bobby's just like the charter boats do. I'd email their spreadsheet a week before my trip and instead of delivering to charter base, I'd ask them to deliver to the Anegada ferry. We would just go to the dock and meet the ferry when it arrived on Anegada. I also flew from STT or SJU so I would have everything delivered by Bobby's; liquor, water, food, etc.