Process Manufacturing Alternate Ingredient Items

We are on 12.1.3 using process manufacturing. As we are a pharmaceutical company, the manufacturer of the raw materials are important, as not all manufacturers may be registered for a product.  We therefore give different item codes if the item is sourced from different manufacturers. 

We therefore may have stock from a specific manufacturer but the formula is setup for the other item code, but this may be very temporary and therefore do not want to go the substitution route where one ingredient replaces another for a period. As you can select an alternate resource we would want to select an alternate item, but only from an approved list for the product.