They used to be found at Engine Ears at DCA but since that has closed there haven't been any reports of the 'stuff a box full of Mr Potato Head parts' which is a shame because they are so cool!
I hope they bring it back! That was a great day we spent building and picking out Mr. Potato head pieces! Our DD had over 6 Mr. Potato heads lined up while we were picking out pieces and the staff showed us how to literally over-stuff the box so we got an amazing amount of pieces! Such a great souvenir!
BRING IT BACK! BRING IT BACK!
PS/ I just did some research on DLR web site and here is what is says about "Engine-Ears":
*Engine-Ears Toys closed 08/01/11 and will reopen 05/30/12 as part of Buena Vista Street