If you can wait until you get to the waterpark, I'd get them there. They make great souvenirs & will be one of your least inexpensive souvenirs as well (about $4.00 for small ones, $6.00 or $7.00 for the large).
They also have a wide assortment of colors with the waterpark logo right on the front. Don't worry that you'll have trouble finding them. The waterpark giftshop has tons!!