How to get x.509 certificate for MongoDB authentication

Hi,

I have worked on setting up MongoDB authentication using x.509 certificates for testing.
In testing, I created self-signed certs using openssl commands.
Source Documentation: https://www.mongodb.com/blog/post/secure-mongodb-with-x-509-authentication
Now I want to move the same configuration to production, so, what kind of certificate should I buy and where can I get that certificate from?
I am on a deadline right now, so, an early response would be appreciated.

Thanks.