Tamara,
While you will find that the Disney Dining Plans are accepted at the majority of the restaurants throughout the Disney property, there are certainly a few that are not on the plan. Downtown Disney is an area where there are a few more of the non-participating restaurants located, so it is always a great idea to print out your respective Dining Plan brochure that lists all the participants.
You can find them on the Walt Disney World website at the
Resorts Benefits page.
As for a place to buy groceries, the easiest to get to is Goodings, which is located just off Disney property by Hotel Plaza Boulevard. It is in the Crossroads Shopping Center, and any cab will know how to get there. If you have your own car, the resort front desk will be able to give you directions. It is really just a short drive away.
Finally, you can also consider using a grocery delivery service. Your resort will be able to recommend a few when you get there.
Have a magical vacation!