The N6850/N8850/N10850 CPU FAN default is JETART SL2060 LGA1155 & 1156 CPU COOLER:
http://www.jetart.com/index.php?do=prod&toppid=43&pid=56&id=211

Here we could purchase JETART SL2060LGA1155 & 1156 CPU COOLER from retail market directly, either or contact with your Thecus reseller, distributor or Thecus office for RMA request directly. And following reseller/distributor/office link for your reference:
http://www.thecus.com/wtb.php
http://www.thecus.com/about_contact.php

If contact Thecus office, please provide the following details by mail :
(Europe) Thecus-RMA@apligo.com
(USA) rma_us@thecus.com
(Taiwan) http://tc.thecus.com/sp_rma.php
1. Serial number
2. Address details
3. Copy of purchasing invoice

For India area please contact Thecus rma_service@thecus.com with following info for RMA number and following process.
1. Serial number
2. Address details
3. Copy of purchasing invoice