A word of caution: make sure your Purchase Orders or Standing Orders contain the complete specifications against which you want the product delivered. If you require Certificates of Compliance or Conformity. make sure they accompany the product when you receive it. When you have placed a Standing Order with a supplier they should submit updated certificates or some other confirmation of compliance each time they make a delivery against that order.
During an ISO9001 assessment at which I was present the audit revealed a certificate that was ten years old, issued for infrequent, long-term delivery of items needed in production. My customer had not asked for certificates of compliance after the first purchase, on the grounds that the supplier was always dependable and the items would be delivered a batch at a time, based on the original Purchase Order. Unfortunately, the assessor did not take this view, and we got a minor non-conformance (the only one, much to my chagrin).