I got ours on Ebay. Searc Kid's Fanny Pack or Disney Fanny Pack. One word of advice....check the pictures. We got a rectangular shaped one with multiple pockets. The ones that are "triangular shaped" don't have as much interior room. A lot of those seem to be PVC plastic too. Look for something more durable. I think we got it for 10 bucks with shipping.
Bought several of these @amazon.com for my son (9 yo) and our friend's children (6yo-9yo) for our trip this past May. The pack itself is on the small side (did not hold an autograph book) but the waist strap was a good size for kids. http://www.amazon.com/gp/product/B000F4SNSM/002-0851066-7798452