How many groceries do you need? If you are only making breakfast, it may work out cheaper to pay the inflated prices in the SSR shop. They have basics like eggs, bacon, cereal, milk, coffee, etc. at convenience store prices.
The shopping services referenced in previous posts are worth considering, but they do charge a fair amount in fees and (optional) tips. They are usually only worthwhile if you're going to buy quite a few groceries.
Taking a taxi to a supermarket is another option, but the cab fare may eat up any savings you'd achieve, plus you're losing time out of your vacation to shop.
Mary
MouseSavers.com