If you mean from the WM fleet directly, I have heard some guys on this forum mention that some of the fleets out west sells them directly. On the east coast I haven't seen any WM DC selling any, but I think you probably have to buy them in bulk before WM even consider you as a factor.
I talked to a senior Walmart driver and he told me if you are looking to purchase one of their trucks you can call a Walmart DC shop and give them the vin # and they can tell you everything that has been done. He said they run synthetic oil and change every 45,000 miles. He said most all trucks had radiators and turbos changed around 600,000. This is what he told me.