Page 15 of the manual says the hardware is included in the shipment. That being the case, I don't believe there's a part number for spare hardware.
Domestic shipments of the NetVanta 1638P include the following items:
• NetVanta 1638P base unit
• NetVanta 500 W PoE power supply
• A detachable power cable with a grounded, three-prong plug
• Quick start guide
• Two 19-inch rack mounting brackets and eight screws
Try part number 1700594G19