I assume you'll be taking the 5S and exiting at Ball Rd./
Disneyland Dr.? If so, go east on Ball to Anaheim Blvd. There is a
WalMart supermarket there where you'll be able to stock up on water and snacks. Also, if you go south on Harbor, about a mile beyond the parks, there is a Target where you can get what you need. Otherwise, you can go straight to your hotel and, after dropping off your car, catch an Uber or Lyft to the 24/7 CVS on the corner of Katella and Harbor (there's a Walgreen's right across the street). The CVS has basic groceries, some produce, dairy, etc.