Hi Sharon:
You will find Disney Dining Plan snacks just about everywhere around Disney World, including in restaurants, in shops, in bakeries, food carts, and in Resort food courts. Snacks will include popcorn, water, muffins, fruit, scones, brownies, cookies, drinks, bottled beverages, coffee, chips, certain side dishes, soups, and chocolate dipped treats. The list of items and locations is too numerous to mention with my word limit, but hopefully that gives you a good idea of your choices.
To confirm Disney Dining Plan snacks, just ask a Cast Member or look for a purple "DDP" square sign on restaurant menus and on stands and countertops in shops.
Enjoy your vacation