I believe you can get one in Epcot at The Land. There is a food court there called the Sunshine Season Food Fair. I remember one of the counters sold baked potatoes. Sorry I can't think of anyplace else.
I just posted today about the GREAT baked potatoes my hubby and I had last time we were in MK. There was a cart just outside of the Hall of Presidents. Definitely the BEST baked potato I've ever had. Butter & salt on the outside & all the fixings you could ask for on the inside!