Your search for "WS-X4991" revealed the following:
NEW Cisco WS-X4991 Catalyst 4900 Series Switch Fan Module
SKU: NSN-9537
Condition: New
Shipping: Overnight Options
Warranty: 365 days
$ 42.99
• Only 2 remaining
Call if you need more
Cisco WS-X4991 Catalyst 4900 Series Switch Fan Module
SKU: NSH-4517
Condition: Seller refurbished
Shipping: Overnight Options
Warranty: 365 days
$ 39.99
• 7 in stock
Call if you need more