The store at SSR has some basics--milk, bread, etc. But they are very expensive to buy there, and the selection is QUITE limited.
If you will have a car, I would suggest simply driving to a nearby grocery store. If you will not have a car, and are using the Magical Express in from the airport, you may choose to use a service like wegoshop. If you won't have a car, but are using another transportation service from the airport, most of those will allow one free "courstey stop" at the grocery store on your way to the resort.
Depending on how much you need to buy, taking a cab to/from a nearby grocery could still be a cost savings over buying at the SSR shop. On the other hand, if all you want are a few small items, the inconvenience may not be worth it to you.
One option that some people use is to pack a suitcase full of nonperishable grocery items (boxed mac & cheese, breakfast cereal, granola bars, that kind of stuff...) and then just buy the perishable things when they get there. That way, you have plenty of suitcase space to bring souvenirs home. I haven't tried this strategy, but I'm thinking about it for our next trip.