Try hitting your local auto parts store, up here in Canada we can get the Dayco replacement steel pulleys over the counter for about $22.00 each(Canadian dollars). They work great and somehow fit the belts better than the Mustang replacements you gte from Ford.
You will not find the correct sized aftermarket pulley for the supercharger, not even from Ford unless you go back to the plastic ones. You end up with all 3 pulleys the same size(the SC one is smaller stock). They work just fine being all the same size.