Agri supply might have what you need. Link below. Getting one with the right concave shape might be an issue. The do list sizes as from 14" to 36".
Malsam Terracing Inc. of Wakeeney, Kansas may also have them if Agri Supply doesn't. Their phone # is 785-743-5542 They are a manufacturer of terracers. Not sure it they make anything other than belt terracers or not.