iProcurement Catalog for Each Inventory Organization

We have 16 inventory organizations and while most share the same items, we do not have all items assigned to each inventory organization and some organizations have unique items which only belong to them. I have setup catalogs/stores for each organization in iProcurement, but since they share the same item master they all have the same item list at the current time. Does anyone know of a way I could restrict the stores to their own inventory organization. The item/PO categories are assigned at the item master, so I can't use them as a way to restrict the items by