As far as I can tell, you can pick up Bus 56 at Ticket and Transportation Center and take it to Osceola Square Mall and Wal-Mart is right across from there.
I've never done it before, but that looks about right from the map.
Just remember, WalMart puts Americans out of real jobs! (I suppose they could work at WalMart, but then they couldn't afford to vacation at WDW.) Please do some research.