As far as I know these USB receivers are coded to the respective mouse &/or keyboard so are irreplaceable.
If it is under the warranty then if you are located in the United States and Canada, you can reach the Microsoft Supplemental Parts team at the contact numbers that are listed here:
United States: (800) 360-7561, Monday through Friday, 5:00 A.M. to 7:00 P.M. Pacific Time.
Canada: (800) 933-4750, Monday through Friday, 5:00 A.M. to 7:00 P.M. Pacific Time.
If you from other parts of the globe then do contact the Microsoft retailer for further help and assistance.
Ref:
How to replace Microsoft software or hardware and order service packs