You can get a certificate from any CA. You can even create a certificate yourself, although you must only use such a certificate for development purposes, not for a live domain controller.
For a production environment, what CA to use kinda depends on who will be accessing the domain controller. For instance, if you only want to use LDAPS within a company intranet, you can setup a CA for your own company, and add its root certificate to the trusted certificates of all systems within your company intranet. Then you request a certificate from your own CA as outlined
here. Install the certificate in the domain controller's personal certificate store. Active Directory will now use this certificate to identify itself when accessing it through LDAPS. Systems in your intranet will authenticate the domain controller's identity by verifying that its certificate was issued by your company's CA, which they trust.
If you need more help, you need to give us details like what kind of applications will be using LDAP, whether the domain controller will only be accessible in the intranet, and if so, why you want to secure it.
