Not sure if you want a design or the actual shirt made. I have this design you can print and iron onto a shirt. Otherwise, I think zazzle has Disney shirts you can customize and order.
I picked up a pack of Hanes T-shirt transfer paper in Wal-mart in the paper section. It came with a CD that helps you make your own transfers. I made one for the front and back of each of our shirts and then printed them out. Made 2 shirts for our trip, chose a bright yellow and a bright green shirt. We all wore the same color shirts on the same day. The colors helped with keeping eyes on the little ones in the crowds!! We received a lot of good comments on them.
Best part for the kids was, everyone picked out their own characters for the front of the shirt.
We found the t-shirts on sale at Michaels, I think it was $2 a shirt when we bought them. Here are 2 pics with the shirts on, I hope you can make out the fronts in them!!