Owner's manual is
here.
The manual does not list the part number for the microphone. The service manual might. Judging by the photo on page 5, you do not want the triangular shaped mics. You want the cylindrical ones. Ebay is your best option. I could not find a listing specific to the TSR-5810. It is long out of production.
OK, found the service manual. Part #WN649600. They have that specific part number on Ebay for $35 plus shipping.
Sample listing here. Make sure it says WN649600.