If the OWN supplier is showing against a sales order line, check the Supplier tab to see if any of the suppliers have a 'Y' in the 'Preferred' tab.
Products that do not have a preferred set will show the OWN supplier when the item is added to a sales order and will need to be updated so that a preferred supplier is set in the stock product record. Click here to see how update all stock products with a preferred supplier.