This is the walmart I used during a BC stay.
Been there on both of my last trips. Very easy to get to but make sure your gps is set to avoid the toll road or carry plenty of change (the road BEHIND the store that gets back to disney, it's a exact change toll road LOL luckily I had the right amount of quarters) however there is a short and easy non toll option as well. Just take the main road in front of the store turning left from the lot. It was maybe 15 minutes from the epcot area.
Wal-Mart Supercenter Store #5420
3250 Vineland Road
Kissimmee, FL 34746
(407) 397-1125
Large, clean, prices are reasonable (I live in midamerica where the price of living is low and while more expensive then home, not unreasonably more), liquor store (i didn't buy so don't know the prices), Subway (makes a great first night dinner to grab a bag of subs and get a chips and soda in the store), they also have a large disney section so a great place to pick up a few inexpensive souvieneers.
I picked up water, beer, milk, chips/snacks and souvineers and spent less than 30 bucks.