The closest big
WalMart to WDW is a newish one at the corner of Osceola Pkwy and Vineland Rd. (aka SR 535). The address is 3250 Vineland Road,
Kissimmee, FL 34746
There are two ways to reach it from the Greenway (417 from the airport).
1) Get off at World Center Drive, exit #6. Coming off the Greenway,
keep straight through the intersection of International Drive, and then turn left onto Vineland Rd (SR535).. Continue on Vineland under the Greenway overpass, past the LBV Factory Store outlet mall (on the left), and you will see the Walmart on your right just before the Osceola overpass, sitting in a triangle intersection with Poinciana Blvd. It is a little over a mile to the store, I think, once you leave the Greenway. This is the shorter route, but that I-Drive intersection can sometimes confuse people.
2) Get off the Greenway at Osceola Parkway (exit #3), but instead of turning right toward WDW, turn left, away from it. The store will be about 2.5-3 miles down, directly off to the left of the road you are on, backing to Osceola in the northeast corner of the intersection of the Poinciana Blvd exit. (Poinciana cuts diagonally between Vineland and Osceola, kind of squares off the corner right next to the store.)
To continue to WDW from the WalMart, the simplest route is to get onto Osceola via the ramp that runs right behind the store. This will take you in in the direction of Animal Kingdom; to reach the rest of the property you would want to turn north on either Victory Way or World Drive once you cross I-4 and get on-property.
It is also possible to go to WDW by going back north on Vineland, then turning left onto World Center Drive (SR536) which changes its name to Epcot Center Drive before it crosses I-4 and enters Disney property. This is the main road to Epcot, which runs past OKW.