Our usual Publix when at BWV is the last one mentioned by Spiceycat, over on International Drive. The Gaylord Palms is such a great landmark, I can never go wrong. I also agree with Spicey, they there is very little traffic on this route, ever.
As for directions, I usually just go to the Valet desk when picking up my car and get a map from them while I wait for my car to come up. But if you want to be absolutely sure ahead of time, you can also plug in the address of the Boardwalk into Yahoo! Maps, then go to a "City" level view, select grocery stores under the shopping option, and get it that way.
The street address of BWV is:
2101 Epcot Resort Blvd, Lake Buena Vista, FL 32830
while our favorite Publix is:
2925 International Dr, Kissimmee, FL 34746
Good luck!