Hi Jodi,
There are Mr. Potato Head stations at Walt Disney World. They used to be in lots places at Disney, but on a recent trip, the only place I can recall seeing them for sure was the Once Upon A Toy store at Downtown Disney.
I have never personally seen a Mr. Potato Head station at a Disney Store in the US, but I've only been to a few of the hundreds out there. If you're interested in a particular geographic area, you may want to call a store directly to confirm. You can use the
Store Locator feature on DisneyStore.com to find the phone number of a particular location.
There are packaged Mr. Potato Head boxes, including some with Disney, Star Wars, and Marvel components available at most big box retailers like Target, Walmart, and Toys R Us, as well as online via Amazon and other retailers. These don't have the "choose your own piece" component, but if you buy a few sets, then you may get much of what you need.
Good luck with your planning. Please let us know what else we can do to help.