If you want a Publix supermarket between MCO and Bay Lake Tower, the one that requires the least extra driving and saves the most time is this one just off 417.
Publix Super Market at Landstar
13850 Landstar Blvd
Orlando
Check the route with Google Maps or Mapquest.
Because 417 is a toll road, you can expect to pay an additional toll (beyond the two $1.50 tolls). I think it will be an extra 50 cents to reenter 417 at Landstar after you shop. Have some quarters ready (if you're not using a transponder or your rental car company's license plate toll service).
From there, it's another 25 minutes to Bay Lake Tower, so bring an insulated bag if you plan to buy any frozen food.
There are bunch of other Publix markets, but they all require bigger detours.