Technically, none, as the fastest route is a toll road: FL417. The most convenient stop when taking 417 is to get off at Osceola Pkwy as you would when heading directly into WDW property, but instead of turning right as you come off the exit, turn left, and go a couple of exits up on Osceola to Poinciana. The WalMart is directly next to the exit on the left.
There is a Target next to I-4 just south of Willis Lake, not far south of the Beachline interchange, but that's deceiving; you can see it, but not get to it; there is no way to get off of I-4 southbound in that area. You'd have to backtrack quite a ways on surface roads to reach it. Also, I-4 is a parking lot more often than not, so it's worth it to pay the toll to take 417.