We just got back from our trip, and our family made 5 sets of matching shirts for our 10 day trip (and I washed on the trip). It was a real blessing, let me tell you! Not only did it make it much easier for us parents to find the kids (DD7 and DS10) in the crowds, but it made it easier for the kids to see us as well. We have found tiedye shirts to be far more visible in crowded conditions than solid colors.
I ordered tiedye shirts from tiedyeusa.com, then customized them with some Disney-themed iron-ons. If you order 24 shirts, you get wholesale prices on the shirts. Our shirts came out to about $5.25 each -- including shipping, which I thought was more than reasonable. I teamed up with a friend to get the 24-shirt minimum order.
BTW, if you buy these tiedye shirts and then decide to dress them up like I did, you probably want to wash them 2-3 times before doing the iron-ons. I only washed them once, and the shirts were still bleeding a fair amount of dye on the second washing.