I still haven't gotten it straight in my head exactly where Sanford is, so I'm not sure what road you'll be taking.
At International Drive and Universal Blvd. there's a very safe 24-hour Walgreen's with an expanded grocery section. They aren't the highest prices around, but they aren't the cheapest, either.
West from I-Drive and from I-4 on Sand Lake Road (about two minutes from either) there's another, cheaper 24 hour Walgreen's, and a 24 hour Albertson's Grocery Store that caters to mostly locals, and is cheap as can be. (The Albertson's is a little hard to see, but it's in the Dr. Phillips Marketplace strip mall, on the northwest corner of the Dr. Phillips/Sand Lake intersection.)
Down Universal or I-Drive (it's between the two) right next to The Mercado is a sometimes 24-hour Gooding's grocery store. They are very unpredictable about whether they'll be open 24 hours or not, and cater exclusively to tourists, so they have a very limited selection of sky-high priced groceries.
I know there are
WalMart Supercenters down closer to WDW, but I don't know their locations, pricing, etc. Perhaps someone else can help you with that?