SSL for Windows where to buy

There are plenty of certification authorities out there....
try
www.digicert.com
or
https://www.rapidssl.com/ 

is a good place to start in my opinion.

To "add it to your web server"

- Generate a CSR within IIS
- paste that CSR into your chosen provider
- you will get a <certname>.cer back
- Complete the certificate request in IIS using that .cer

https://LetsEncrypt.org - Free simple + wildcard certs supported by all browser + mail clients.

https://LetsEncrypt.org - Free simple + wildcard certs supported by all browser + mail clients.

I don´t understand is this really free CA ?

I can't find out how to get it and install

Well the first step would be a windows ACME v2 compliant client.
https://letsencrypt.org/docs/client-options/

Once installed make sure you're using the LetsEncrypts "staging environment" during configuration and testing
https://letsencrypt.org/docs/staging-environment/

When you have things working, change over to production

Here the List of Following websites where you can Buy your SSL certificates

https://www.digicert.com/buy-ssl-certificates/

Know How to Import SSL server
Launch MMC session. ...
Add/Remove Snap-in.
Add the  Certificates.
You have to Use the Computer Account.
You have to Manage the Local Computer.
Expand to the Personal Certificate store.
Now perform Right-click, All Tasks, Import.

Hi,
You have two options, either you can go for let's encrypt which a free SSL certificate and other one is you can purchase from any of the SSL providers such as GlobalSign, DigiCert, GoDaddy etc. its a paid on yearly basis, You may get exciting offers while you purchase.

Let'sencrypt provides you SSL certificates for 90 days and it gets renewed automatically once it reaches the expiry date. To performing this there is a task scheduler checks the expiry date of the certificates.

Here is the link to do configuration for let's encrypt in windows server,
https://www.baswijdenes.com/portfolio/how-to-use-lets-encrypt-certificates-for-windows-servers/

Here is the link for the normal SSL certificate,
https://www.digicert.com/util/csr-creation-SSL-installation-windows-server-2012.htm