I always see some in the corners near the food courts and counter service places in the parks........if you dont see one ...ask a cm to see if he or she can locate one that may of been stashed in a different area from a family that is all thru using it
We have never had trouble finding a highchair anywhere in the world. As PP suggested, if you don't see on or are having trouble managing your toddler, food and the high chair just ask a CM and they will be happy to bring one to your table.
the only place i can think of that we have had trouble finding one is the pinnochio village haus in the MK but overall they are usually pretty easy to dig one up