How to export certificate from Domain controler

Use the the following command to generate certificate.

C:\>certutil -ca.cert certificatename.crt

you will find your generated certificate in C:\
please note that Active Directory Certificate Services will be installed on your Domain controller, you can change the certificate extension according to your requirements.

Free SSL certificate

https://ssl.comodo.com/free-ssl-certificate.php
https://www.instantssl.com/free-ssl-certificate.html