I had purchased this headband from PricePoint to keep my ears warm in the winter; turns out it makes a great sweat band also. They just redesigned it with a cutout in the back for ponytails, which apparently called for an extra two dollars, but it's the same material. They go on sale every few months, so you should be able to get it for about five bucks.
http://www.pricepoint.com/detail/201...t-Headband.htm