We found the best place to get the Mr. Potato Head was at the toy store at DTD. Had the largest variety of 'accessories'. Also - don't forget to stuff Mr. PotatoHeads butt, LOL. A lot of people were just tucking a few things in the box around him and the check-out gals would then tell them they could get more inside!!
DD basically make a selection of all the trinkets she wanted and then we sat down over near the edge of the room and proceeded to make sure they all fit inside that potato and the box!!! It was a blast. We spent a long time doing it one evening and had a great time. She then 'dressed' him/her each morning with things that went with what we were doing that day.