Christianlanequilters.com has the trademark for a 'fatback'. All others must call themselves wide backs.
sewbatik.com is also where you can get wide backs. Use LAP1250 in instruction box so they know you heard it from me. I've actually used their product. Most batiks are too tightly woven to be used without problem in a back - the threads break instead of part for the needle. These wide backs are made from the same kinds of goods that quilting fabric is made from